Index
The PICAXE Telephone Exchange Simulator
Important Note - Limitations on Use - Prohibitions
The Exchange Simulator Prototype
Complete Build Plans
Theory of Operation
Electronics
Veroboard Layout and Track Cutting Diagram
Software
Programming the PICAXE
Using Windows 98 SE as a Dial-Up Networking Server
Making It All Work
The Dreaded Dial-Up Networking Crash
Appendix 1 - Project Journal
Appendix 1 - Project History
Appendix 3 - MS-DOS Modem Emulator Software
Appendix 4 - Establishing Line Level Settings
The PICAXE Telephone Exchange Simulator
The Telephone Exchange Simulator is designed to allow connection of any modem
device which normally connects to an ISP or other service provider to connect
with a local PC through an external modem.
It is a low cost, DIY design which should take just a few hours to construct
and get working.
There are many devices which connect to an Internet Service Provider (ISP)
through an in-built modem to obtain access to particular services. Games
machines such as the Dreamcast connect to online games servers and Internet TV
Set-Top Boxes allow web surfing on the household television.
In many cases, such units could equally connect to an in-house PC acting
as an ISP, to either bridge the dial-up connection to a high-speed broadband
internet link, or to deliver the required services locally.
Pretending to be ( or "emulating" in professional terms ) an ISP is not hard;
in most cases all that is needed is a modem and some software which can handle
the incoming calls to the modem. Windows, Linux and many other operating systems
can act as dial-up network servers, although the facilities offered will depend
upon the operating system itself and what other software is installed.
It is the ability to connect one modem to another which presents the problem.
You can't connect them back-to-back, one wired directly to the other, because
they need to telephone network coneection to make them work.
If you have two independent telephone lines you can dial out on one line to call
back in on the other, and it works fine, except that it costs money and isn't a
particularly elegant solution, tying up two household telephone lines.
If you have access to an office telephone exchange ( a PBX or PABX ), two
separate extension lines can be used to keep the entire inter-connection local,
but few households have their own telephone exchanges, they are not cheap to
purchase, and they are often overkill for what is needed.
This project is to create a simple telephone exchange which is specifically
designed to act as a two line mini telephone exchange, and most importantly
of all, to be cheap and easily constructed.
The project is called a Telephone Exchange Simulator because it is not actually
a telephone exchange, in fact it is limited to one particular purpose, but to
a device that dials-out and expects to be connected to a genuine telephone
exchange, it will appear to be one.
Such Exchange Simulators may sometimes be referred to as Telephone Line
Simulators but a Line Simulator usually provides dial and other tones, ringing
signals and is often designed to be indistinguishable from a public telephone
exchange, and as this does none of that, it is presented under the description
of an Exchange Simulator.
Important Note - Limitations on Use - Prohibitions
|
|
Under no circumstances must the Telephone Exchange Simulator described here
be connected to a public telecommunications system in any way. Doing so will
render you liable to prosecution and other penalties. Connection to a public
telecommunications system or any private telephone exchange will probably
destroy the electronics of this device and may damage the telephone
exchange.
|
The Exchange Simulator is a standalone unit used to connect two standalone
modem devices within your own property and must not be used for any
other purpose.
The electronics are incompatible with the voltages used and generated by other
public and private telephone systems and the Exchange Simulator must not be
used in any manner or for any purpose other than described within this document.
The Exchange Simulator Prototype
 |
|
At the heart of the Exchange Simulator is the well specified PICAXE-08M, shown
in this prototype which differs slightly from the layout presented later.
Connecting it all up would probably benefit from having a greater choice of
coloured wire to hand.
|
Complete Build Plans
Complete build plans; circuit diagrams and veroboard layout and track cutting
diagrams are available on a single page for convenient printing. Please see
the following page for details ...
PICAXE Telephone Exchange Simulator Build Plans
Theory of Operation
The project is really an attempt to build a telephone exchange in miniature.
Although full-blown public and private telephone exchanges are extremely
complex, they are based upon incredible simple principles which go all the way
back to the very earliest 'plug-in a cord' manual routing exchanges. Although
telephone technology has moved on, phones and modems are still compliant with
those principles.
A very simple telephone voice link can be established with just a power supply,
and a resistor ...
-.- +12V
|
.--------------. .|. .--------------.
.-| .--------. | 330R | | | .--------. |-.
| `--' `----' `--' |_| `--' `----' `--' |
\ | [] [] [] | | | [] [] [] | /
/ | [] [] [] | .---O )----^----( O---. | [] [] [] | \
\___| [] [] [] |===={ }====| [] [] [] |___/
| [] [] [] | `---O )----.----( O---' | [] [] [] |
`----------' | `----------'
-^- 0V
This is about the simplest 'telephone exchange' one could build; when both
handsets are lifted ( 'off-hook' ) users of both telephones can talk to each
other. As the resistance on one side of the circuit increases and decreases
during speech, the voltage change in the potential divider is reflected in the
level of sound produced on the other. It's a bit more involved than that, but
that's the general principle, and will suffice here.
Because modems are pretty much telephones hidden inside a box, we can quite
easily use the same circuit to connect two modems back-to-back ...
-.- +12V .-----.
| | PC |-----.
.---------------. .|. | | |
| Dial-Out | 330R | | .-^=====^-. |
| Device | |_| `---------' |
| | | |
| .---------. | .---O )----^----( O---. .---------. |
| | Modem |======{ }===| Modem |---'
| `---------' | `---O )----.----( O---' `---------'
`---------------' |
-^- 0V
The voltage supply is reasonably non-critical and +12V seems to
work okay, with the current drawn being below 30mA. Although it would
be nice to use a +5V supply ( convenient for other electronics we will be
adding later ), such attempts have not proved successful.
This system works and can be used, but it requires manual intervention on the PC
to establish the communications link. The big problem with the simple system
described so far is that there is no way for one party to tell the other to
automatically pick-up and answer an incoming call. Real telephone exchange
systems put a reasonably high voltage AC signal down the line which the
telephone detects and uses to ring its bell or activate its warbler. A modem
uses the very same signal to detect an incoming call and to send the "RING"
message to a PC when an incoming call occurs, and the PC then tells the modem
to pick-up and answer the call. Once the call is established, the communications
are all under the control of the devices at each end of the line.
Although we do not want to have to add a high voltage AC signal generator to our
circuit it is important to see how it would be activated if it were present.
When the Dial-Out Device is idle, it will keep its modem 'on-hook' and draw
minimal current from the interface. When it dials out it will go 'off-hook',
present a resistance and draw current from the supply. The current drawn can
be used by a telephone exchange to detect when a caller has gone off-hook and
wishes to place a call and can subsequently be used to route the ringing signal
to the required destination.
We can add our own off-hook detection by using a PICAXE microcontroller
manufactured by Revolution Education Limited
to monitor the analogue voltage of the line linking the two modems together ...
-.- +12V -.- +5V
.|. |
330R | | .---^---.
|_| 18K | 5V | .-----.
| ___ | | | PC |-----.
}----|___|---.--->| A2D | | | |
| .|. | | .-^=====^-. |
| | | | 0V | `---------' |
.---------------. | 10K |_| `---.---' |
| Dial-Out | | | | |
| Device | | }--------' |
| | | | |
| .---------. | .---O )----^------------|---------( O---. .---------. |
| | Modem |======{ | }===| Modem |---'
| `---------' | `---O )----.------------^---------( O---' `---------'
`---------------' |
-^- 0V
The 18K and 10K resistors form a potential divider dropping the +12V line
voltage down to 4.2V, below the +5V the PICAXE is powered from. If a higher line
voltage is used, the resistor divider will need to be altered to not exceed
the maximum A2D voltage allowed which would destroy the PICAXE.
We are now able to detect when the Dial-Out Device has gone off-hook, and must
consequently issue a ringing signal to the PC modem. In reality, we don't; all
the modem does when waiting for an incoming
call, is to work as an AC ringing voltage detector issuing the "RING" message
to the PC when it occurs. If the PICAXE can send the PC that "RING" message it
will be unaware that it didn't come from the modem, will assume it has and will
consequently tell the modem to pick-up the incoming call and neither are none
the wiser.
Fortunately the PICAXE can easily interface to a PC serial line, and can simply
be wire-or'd by diode-mixing to the very line that the modem communicates to
the PC with. This makes presents an ideal solution, but does require an external
modem to be used ...
-.- +12V -.- +5V
.|. |
330R | | .---^---. .-----.
|_| 18K | 5V | | PC |=====.---.
| ___ | | | | | |
}----|___|---.--->| TX |--. .-^=====^-. | |
| .|. | | | `---------' | |
| | | | 0V | | | |
.---------------. | 10K |_| `---.---' | |\ | | |
| Dial-Out | | | | `------| >|-------{ |
| Device | | }--------' |/ | _|_ |
| | | | /_\ |
| .---------. | .---O )----^------------|---------( O---. .---------. | |
| | Modem |======{ | }===| Modem |==='---'
| `---------' | `---O )----.------------^---------( O---' `---------'
`---------------' |
-^- 0V
Note that the Exchange Simulator is one way only; it will only handle calls made
from the Dial-Out Device to the PC, and not the other way round. Attempting to
initiate a call out from the PC will trigger the off-hook detection and cause
a "RING" message to be sent back to the PC which is likely to confuse the
dialling software, and quite possibly corrupt the actual modem responses.
In normal use the potential for the "RING" message to collide with modem
responses is minimal as the modem sends nothing when waiting for an incoming
call. The worse case would be when the Dial-Out Device hangs up and attempts
to place another call while the PC is re-initialising the modem; the Dial-Up
Networking Server software should however be able to untangle the mess. It
would also be possible for the PICAXE to ignore any off-hook transitions for
a time after both go on-hook which will effectively ignore the problem; the
Dial-Out Device should report its inability to connect during that time.
Electronics
The circuit uses a PICAXE-08M and its Serial Out connection ( leg 7 ) is used
to communicate with the PC. The PICAXE-08M has a huge advantage over the
PICAXE-08; it can execute at 8MHz, and permits serial communications of up to
9600 baud.
BT431A -.- +12V BT431A
Line .|. Line
Socket | | 330R Socket
.------. |_| .------.
1 | =====| | |===== | 1
2 | =====|-------^-----------.----------------------------------------|===== | 2
3 | =====| | ___ ___ |===== | 3
4 | =====| `---|___|---.-------|___|--------. |===== | 4
5 | =====|-------. +5V -.- 18K | 10K | .---|===== | 5
6 | =====| | | | | | |===== | 6
`------' | | .---..---. | | | `------'
Dial-Out | `---| +V 0V |---|--------------------{ | Modem
Device | .---| SI O0 |---|---. | |
| .|. | D4 A1 |---' | |\ | 22K | |
.-. | 22K |_| | I3 D2 |---. `--| >|--.---. | |
Download |O|--------|------{ `--------' | |/ | | .|. | |
.-|O|-----. | .|. PICAXE-08M | 1N4001 | | | | |
Run `-|O|--. | | 10K |_| | | |_| | |
`-' | | | | | | | | |
Molex | | }------^----------------|------------|---^---^---'
link | | | | /| ___ | |\ | |
| | }------|< |----|___|----' .--| >|--{
.---. | | | | \| 1K8 | |/ | | .---.
| \ | | | LED | 1N4001 | | \
1 | O---\--|--|--|---------------------------|--------|-----|-O \ 1 DCD to PC
6 | O-|--|--|--|---------------------------|--------|-----|---O | 6 DSR to PC
2 | O---|--|--|--|---------------------------' `-----|-O | 2 TX to PC
7 | O-|--|--|--|------------------------------------------|---O | 7 RTS from PC
3 | O---|--' `--|------------------------------------------|-O | 3 RX from PC
8 | O-|--------|------------------------------------------|---O | 8 CTS to PC
4 | O---|--------|------------------------------------------|-O | 4 DTR from PC
9 | O-|--------|------------------------------------------|---O | 9 RI to PC
5 | O---/--------^---.--------------------------------------|-O / 5 GND
| / | | /
`---' -^- 0V `---'
9-Way Male D 9-Way Female D
From Modem From PC
Although not an absolute necessity, an additional 'Diagnostics Port' can be
added which allows a PC or other device to monitor the Exchange Simulator's
operation through a serial line. The Diagnostics Port can be used to help
configure the Exchange Simulator should the default software not operate
reliably or correctly with its default line voltage monitoring settings.
.---.
| \
.-------|-O \ 1 DCD to PC
PICAXE }-------|---O | 6 DSR to PC
'D4' >-----|-------|-O | 2 TX to PC
| .---|---O | 7 RTS from PC
| | | O | 3 RX from PC
| `---|---O | 8 CTS to PC
`-------|-O | 4 DTR from PC
| O | 9 RI to PC
.-------------|-O / 5 GND
| | /
0V -^- `---'
9 Way Female D
To PC
Using a PICAXE-18X
Unfortunately, the PICAXE-08M had not been released when this project was
jump-started into life, but the 08M can be replaced by an 18X which has the
same capabilities as the 08M.
The PICAXE-18X can be wired to act as the PICAXE-08M as shown in the following
diagram ...
.--------------------. .---------------------.
| `----' |
| |
__| |__
+V |__|-------.-----------------------. .-----------|__| 0V
| | .------------------|---|-------. |
| .|. | PICAXE-18X | | | |
| 4K7 | | | .----..----. | | | |
__| |_| | | A2 A1 |---|---|---. | |__
SI |__|---. | `---| SO A0 | | | | `---|__| O0
| `---|--------| SI I7 | | | | |
| `--------| RST I6 | | | | |
| .--------| 0V +V |---' | | |
__| | | O0 O7 | | | |__
D4 |__|---. | | O1 O6 | | `-------|__| A1
| | | .---| O2 O5 | | |
| | | | | O3 O4 |---. | |
| | | | `----------' | | |
__| | | | | | |__
I3 |__| | | `------------------|---|-----------|__| D2
| | `-----------------------|---' |
| `---------------------------' |
| |
`-----------------------------------------------'
The PICAXE-08M software will run without change on a PICAXE-18X wired as shown
in the above diagram.
Using a PICmicro
Any PICmicro which is pin compatible with the PICAXE-08 or 08M would be usable,
but software development would be more complex, and beyond the scope of most
people who aren't programmers, and programming a PICmicro requires more than
just a serial cable - The ease of use of the PICAXE development system means
the PICAXE can be used by anyone without any programming experience, and is
cost free beyond the PICAXE-08M purchase.
The advantage of using a PICmicro is of course much better control over what the
software is doing, in particular the ability to code serial communication
routines which run at higher baud rates, but the penalty is in having to have
the necessary skills to achieve that. It is a valid choice to use a PICmicro
for those who can, but it is currently outside the scope of this present design.
UK Line Socket Pinout
The following diagram shows the UK Line Socket pinout when viewed looking into
the socket, with the pins at the top. The UK Line Socket and wiring standard
is used by all domestic and commercial telephone service providers within the
UK.
Most Line Sockets will have six pins, but some may only have four. If there
are only four pins, they will be the middle ones, pins 5, 4, 3 and 2.
6 5 4 3 2 1
.--------------------.
| \/ \/ \/ \/ \/ \/ `.
| .'
`--------------------'
UK Line Socket - BT431A
The wiring connections in the circuit diagram are those used within the UK
and may need to be changed for use in other countries.
Power Supply
The +12V used for providing power to the telephone line sockets needs to be
fairly well regulated and the following power supply circuit is recommended ...
.--------------------------> +12V
7812 | 7805
|\ | .---------. | .---------.
VUR >---| >|---.-------| I O |----.---^-----| I O |----.-----> +5V
|/ | | | GND | | | GND | |
__|__ `----.----' __|__ `----.----' __|__
===== 4700uF | --.-- 100nF | --.-- 100nF
| 25V | | | |
0V >----------^------------^---------^--------------^---------^-----> 0V
The unregulated input voltage (VUR) can be AC or DC, and should create +15V or
slightly more at the inputs of the 7812 and 7805 regulators. The 7V drop across
the 7805 regulator is not a problem, as the PICAXE part of the circuit draws
very little current. Both regulators can be of the 100mA type and don't
require heatsinks.
A Power-On LED ( not shown in the above diagram ) can be added, powered through
a 4K7 resistor connected after the rectifying diode.
Parts List
1 x PICAXE-08M
1 x 8-Pin DIL Socket
1 x 7812 or 78L12 +12V Regulator
1 x 7805 or 78L05 +5V Regulator
4 x 1N4148 Signal Diodes
1 x Red LED ( Power On )
1 x Green LED ( Status )
1 x 4700uF/25V Electrolytic Capacitor
2 x 100nF Polyester Capacitor
1 x 330R Resistor
1 x 1K8 Resistor
1 x 4K7 Resistor
2 x 10K Resistor
1 x 18K Resistor
2 x 22K Resistor
1 x 3-Pin 0.1" Molex Terminal
1 x 2-Pin 0.1" Molex Link
1 x 9-Way Female D Socket
1 x 9-Way Female D Socket ( Optional : Diagnostic Port )
1 x 9-Way Male D Socket
2 x UK Single Telephone Line Sockets
Veroboard Layout and Track Cutting Diagram
The Veroboard ( Stripboard ) layout I used is shown below. It may not be the
best layout possible, but comes quite close. The board is reasonably densely
packed and was entirely laid out by hand. There are no 'dubious connections'
and it is quite straight forward to assemble, requiring only a few track cuts.
The veroboard has its tracks running horizontally, has 12 tracks, and 20
columns; 1.2" x 2.0".
The following diagrams shows the component placements and track cuts required as
seen from the component side of the board. When tracing the tracks from the
component placement diagram, all tracks are continuous except under horizontally
mounted components where there is an implied track break ( explicitly shown in
the track cutting diagram ), and where a track cut is explicitly marked with a
yellow "X".
The solid red and solid grey wires connecting to the left of the board are
the two power supply inputs; VUR and 0V respectively. The shaded yellow wire is
the diagnostic serial line output which if used should connect to Pin 2 of
a Female D connector, with the shaded grey wire, its corresponding 0V,
connected to Pin 5 of the same connector.
Software
The PICAXE controlling software is quite simple; at turn-on the Exchange
Simulator waits until the modem that will be initiating the call is on-hook,
waits for an off-hook detection, sends a "RING" message, and repeats the cycle.
The LED is used to indicate the status; the status indications are described
later.
Note that five lines of the source code have been split over consecutive lines
to make the program more readable; these lines will have to be concatenated to
single lines in order to successfully compile the program if you cut and paste
the code presented here into the Programming Editor.
' ***************************************************************
' * *
' * PICAXE-08M Telephone Exchange Simulator PICAXELS.BAS *
' * *
' ***************************************************************
SYMBOL A2D = 1
SYMBOL LED = 2
SYMBOL TX = 4
SYMBOL LED_PIN = pin2
SYMBOL ledStatus = w0 ' b0/b1
SYMBOL lineLevel = b2
SYMBOL minLineLevel = b3
SYMBOL maxLineLevel = b4
SYMBOL onHookCount = b5
SYMBOL offHookCount = b6
SYMBOL powerOnDelay = b7
SYMBOL ringDelay = b8
SYMBOL MIN_ON_HOOK_LINE_LEVEL = 224
SYMBOL MAX_OFF_HOOK_LINE_LEVEL = 152
SYMBOL LINE_LEVEL_GAP = MIN_ON_HOOK_LINE_LEVEL -
MAX_OFF_HOOK_LINE_LEVEL
SYMBOL LINE_LEVEL_OFFSET = LINE_LEVEL_GAP / 3
SYMBOL ON_HOOK_LINE_LEVEL = MIN_ON_HOOK_LINE_LEVEL -
LINE_LEVEL_OFFSET
SYMBOL OFF_HOOK_LINE_LEVEL = MAX_OFF_HOOK_LINE_LEVEL +
LINE_LEVEL_OFFSET
SYMBOL ON_HOOK_THRESHOLD = 5
SYMBOL OFF_HOOK_THRESHOLD = 5
SYMBOL LED_STATUS_POWERUP = %0100000000000000
SYMBOL LED_STATUS_UNKNOWN = %0000000011111111
SYMBOL LED_STATUS_ERROR = %0011001100110011
SYMBOL LED_STATUS_RING_DELAY = %0011111111111111
SYMBOL LED_STATUS_ON_HOOK = %0000000000000000
SYMBOL LED_STATUS_OFF_HOOK = %1111111111111111
SYMBOL TX_BAUD = N2400 ' 4800 @ 8MHz
PowerOnReset:
SETFREQ M8
LOW LED
ledStatus = LED_STATUS_POWERUP
FOR powerOnDelay = 0 TO 100 ' 5s
GOSUB UpdateLedAndCheckLineLevel
NEXT
SEROUT TX,TX_BAUD,("= ",#OFF_HOOK_LINE_LEVEL,
"/",#ON_HOOK_LINE_LEVEL,CR,LF)
onHookCount = 0
offHookCount = 0
minLineLevel = 255
maxLineLevel = 0
ledStatus = LED_STATUS_UNKNOWN
UnknownState:
GOSUB UpdateLedAndCheckLineLevel
IF onHookCount >= ON_HOOK_THRESHOLD THEN GoneOnHook
IF offHookCount < OFF_HOOK_THRESHOLD THEN UnknownState
ledStatus = LED_STATUS_ERROR
SEROUT TX,TX_BAUD,("Stuck ")
GOTO IsOffHook
GoneOnHook:
ledStatus = LED_STATUS_ON_HOOK
IsOnHook:
SEROUT TX,TX_BAUD,("On",CR,LF)
OnHook:
GOSUB UpdateLedAndCheckLineLevel
IF offHookCount < OFF_HOOK_THRESHOLD THEN OnHook
GoneOffHook:
ledStatus = LED_STATUS_RING_DELAY
IsOffHook:
SEROUT TX,TX_BAUD,("Off",CR,LF)
IF ledStatus <> LED_STATUS_RING_DELAY THEN OffHook
FOR ringDelay = 0 TO 100 ' 5s
GOSUB UpdateLedAndCheckLineLevel
IF onHookCount >= ON_HOOK_THRESHOLD THEN GoneOnHook
NEXT
ledStatus = LED_STATUS_OFF_HOOK
SEROUT TX,TX_BAUD,("Ring",CR,LF)
SERTXD("RING",CR,LF)
OffHook:
GOSUB UpdateLedAndCheckLineLevel
IF onHookCount < ON_HOOK_THRESHOLD THEN OffHook
GOTO GoneOnHook
UpdateLedAndCheckLineLevel:
PAUSE 100 ' 50mS @ 8MHz
LED_PIN = bit15
ledStatus = ledStatus * 2 | bit15
IF powerOnDelay <= 100 THEN InPowerUpDelay
READADC A2D,lineLevel
GOSUB UpdateOnAndOffHookCounts
GOSUB CheckMinAndMaxLineLevelChanges
InPowerUpDelay:
RETURN
UpdateOnAndOffHookCounts:
IF lineLevel >= ON_HOOK_LINE_LEVEL THEN GotOnHookLineLevel
IF lineLevel <= OFF_HOOK_LINE_LEVEL THEN GotOffHookLineLevel
RETURN
GotOnHookLineLevel:
onHookCount = onHookCount+1
offHookCount = 0
RETURN
GotOffHookLineLevel:
offHookCount = offHookCount+1
onHookCount = 0
RETURN
CheckMinAndMaxLineLevelChanges:
IF lineLevel >= minLineLevel AND
lineLevel <= maxLineLevel THEN NoMinOrMaxLineLevelChange
CheckMinLineLevel:
IF lineLevel >= minLineLevel THEN CheckMaxLineLevel
minLineLevel = lineLevel
CheckMaxLineLevel:
IF lineLevel <= maxLineLevel THEN ReportMinAndMaxLineLevels
maxLineLevel = lineLevel
ReportMinAndMaxLineLevels:
SEROUT TX,TX_BAUD,(#minLineLevel,"..",#maxLineLevel,CR,LF)
NoMinOrMaxLineLevelChange:
RETURN
In addition to providing the Telephone Exchange Simulator functionality, the
program also provides a diagnostics output on its 'D4' pin ( leg 3 ) which
can be used to monitor the operation of the program through an additional serial
line.
The diagnostic information is delivered at 4800 baud, and consists of messages
to indicate when the modems go on and off-hook and the minimum and maximum Line
Levels read which can be used to set the MIN_ON_HOOK_LINE_LEVEL and
MAX_OFF_HOOK_LINE_LEVEL constants - see
Appendix 4 for
further details.
LED Status Indication
The red LED is on when power is applied to the Exchange Simulator. The green
LED is used to display the Exchange Simulator's status ...
LED blips-on
The Exchange Simulator has been powered-up and is waiting for line level
voltages to stabilise. This will normally last around 5 seconds.
LED flashing quickly
The Exchange Simulator has powered-up, and it appears that the dialling-out
modem is off-hook. This is an 'error condition' because the PC will not
receive any "RING" message. The error condition is cleared when the dialling-out
device puts its modem on-hook.
LED off
The dialling-out device has its modem on-hook. This is the normal state when
the dial-out device is turned off or has not dialled out.
LED on, blips-off
The Exchange Simulator has detected that the dial-out device has taken its modem
off-hook, and there is an approximately 5 second delay before it issues the
"RING" message to the PC.
LED on
The dialling-out device has its modem off-hook, the "RING" message has been
sent to the PC. This is the normal state when the dial-out device has called the
PC.
Diagnostics Information
The information presented as 4800 baud serial data through the Diagnostics Port
is as follows ...
= off/on
This message is presented whenever the Exchange Simulator is powered-up. Both
'off' and 'on' are decimal numbers indicating what values the
OFF_HOOK_LINE_LEVEL and ON_HOOK_LINE_LEVEL constants have in the program being
executed.
Stuck Off
This message indicates that when the Exchange Simulator was powered-up it
detected that the dialling-out device's modem was already off-hook.
On
This message indicates that an off-hook to on-hook transition was detected; the
modem is on-hook.
Off
This message indicates that an on-hook to on-hook transition was detected; the
modem is off-hook.
Ring
This message indicates that a "RING" message has been sent to the PC.
min..max
Both 'min' and 'max' are decimal values indicating the minimum and maximum
line level values seen since the Exchange Simulator was poweed-up.
Programming the PICAXE
The PICAXE is programmed through the same serial port connection used to link
to the PC when operating as an Exchange Simulator. To enable programming, the
Molex Link must be moved to the position furthest from the PICAXE which
also disconnects the serial connection to the modem to prevent interference
during downloading. If there are any problems during downloading; disconnect
the modem's serial cable from the Exchange Simulator.
The PICAXE can be programmed using the same PC serial port as is used during
operation or through any other serial port. If the port is the same as that used
during normal operation, the Dial-Up Networking Server must be disabled to
allow access to the serial port.
To perform the programming, the
PICAXE Programming Editor
must be launched, the View then Options menu option should be
clicked and the processor type set to program a PICAXE-08M at 4MHz, and the
correct serial port must be selected.
The program to be executed by the PICAXE should be typed, copied, or
loaded ( if already saved to disk ), into the Programming Editor, and
downloaded using the PICAXE then Run menu option.
If the program does not download; check you have selected the correct PICAXE
processor and operating speed, the correct serial port, and have powered the
Exchange Simulator on, and try again.
If the program still does not download; power-down the Exchange Simulator,
disconnect the modem's serial cable from the Exchange Simulator, attempt the
download again, turning the Exchange Simulator on when the "Looking for
hardware" pop-up appears. In PICAXE circles, this is known as a "Hard Reset".
When programming is complete, the Exchange Simulator should be powered-down and
the Molex link moved to the position nearest the PICAXE to enable normal
operation.
Using Windows 98 SE as a Dial-Up Networking Server
I, and most of those of my friends who use the Windows operating system at home,
still use Windows 98 Second Edition - primarily because of its far superior
MS-DOS support to that offered in Windows XP. For that reason I am only able to
explain configuring the Dial-Up Networking Server ( DUN Server ) as used on a
Windows 98 SE based PC, but the principles should be the same for other
Windows operating systems.
The PICAXE Telephone Exchange Simulator was originally envisaged to allow a
Bush Internet IBX Set-Top Box to connect to a PC so
pages served from a local web server could be delivered to the STB. The actual
implementation is a bit more complex than that, and uses an own-designed web
server to provide a great deal more additional functionality than a traditional
web server does. The description here will be limited to getting a Bush STB
displaying pages from a standard PC web server, then displaying pages from the
internet using the PC as a bridge to an ISP.
The instructions here are given as guidance to people who are unfamiliar
with Dial-Up Networking Services but cannot hope to cover all eventualities and
possibilities. The instructions should however be sufficient to get most people
up and running. If you are running firewall software, then this may need to be
re-configured or disabled to get Dial-Up Networking to function as required.
Installing a PC Web Server
If you do not have a web server already installed on your PC you will need to
install one. If entering http://127.0.0.1 into
your web browser address field returns a "Page not found" or "No such server"
error message, you either don't have a web server installed, it is not running,
or it hasn't been configured correctly.
If you have a web server installed, configured and running correctly, then it
should display a home page ( most likely to be index.htm ). If you know
you have a web server installed but the page or server is not found then you
need to configure it correctly.
If you don't have a web server installed, then you need to get one. There are
numerous web servers available for Windows, many of which are superb pieces of
software and entirely free; the web server to be installed here is the
SimpleServer:WWW
from AnalogX, not because it is necessarily
the best or most functional, but because it is free, a small download, and is
very easy and quick to configure. It is a very competent offering and is well
suited to what we are attempting to do. The web server can be replaced
by another at a later time if it is so desired.
Installing the AnalogX SimpleServer:WWW
You will need to download the SimpleServer:WWW installation from the following
page ...
Create a C:\DUNWWW directory and download the SSWWWI.EXE executable to it; it is
about 200KB and should be a fairly quick download.
When the download completes, run the C:\DUNWWW\SSWWWI.EXE executable. Follow the
on-screen instructions and the web server will be installed.
Configuring the AnalogX SimpleServer:WWW
Use Windows Notepad or some other text editor ( not a word processor ) to create
a file with the following contents ...
<HTML>
<BODY>
<SCRIPT LANGUAGE="JavaScript">
document.writeln( "Yippee - " + location.host )
</SCRIPT>
<P>
<A HREF="http://news.bbc.co.uk">BBC News</A><BR>
<A HREF="http://www.google.com">Google</A><BR>
</BODY>
</HTML>
Save the file as INDEX.HTM in the C:\DUNWWW directory - Remember to select
"Save as type : All Files (*.*)" or the file will be given a .TXT filename
extension not the .HTM it needs to have.
Use the Start Menu and through Programs, AnalogX,
SimpleServer and WWW launch the SimpleServer.WWW
application - Take care to launch the right application !
Take a look at the top 'button' and make a note of what it says; mine says
"http://10.0.0.10/" but yours will probably be very different. The information
is needed to test the web server and when we come to configure the Bush
Internet Set-Top Box later.
Click the Click here to set web directory button and locate the
C:\DUNWWW\INDEX.HTM file. Click OK, then click the Start button of
the SimpleServer:WWW application. The web server will now be running.
Enter http://127.0.0.1 into your web browser's
address field ( or click the link ), and, after a short delay, you should see
the "Yippee" message you placed in the C:\DUNWWW\INDEX.HTM file displayed,
followed by "127.0.0.1".
Repeat the process again using the information you wrote down earlier in the
address field ( in my case "http://10.0.0.10" ); this should also display the
"Yippee" message followed by the numbers specified in the address bar.
Installing a PC Proxy Server
A proxy server acts as a bridge between a device trying to get to the internet
and the internet itself; the device which wants to obtain a web page asks the
proxy to go and fetch it for it and then hand it over. This avoids all the
complexities of what is known as "routing", although it does not offer the same
level of capabilities as a proper software router would. A proxy server is
however good enough to demonstrate the capabilities that can be enabled.
The proxy server we shall be using is another free offering from
AnalogX, again, not because it is necessarily
the best or most comprehensive but for the very same reasons we used their web
server; because it is free, a small download, is very easy and quick to
configure and does the job it needs to.
Installing the AnalogX Proxy Server
You will need to download the proxy server installation from the following
page ...
Download the PROXYI.EXE file into the C:\DUNWWW directory we created for the
SimpleServer:WWW; it is about 300KB and should be another fairly quick download.
When the download completes, run the C:\DUNWWW\PROXYI.EXE executable. Follow the
on-screen instructions and the proxy server will be installed.
Configuring the AnalogX Proxy Server
Use the Start Menu and through Programs, AnalogX and
Proxy launch the Proxy application - Take care to launch the
right application !
Ignore the warning message for now, click OK. If the proxy server
minimises itself to the System Tray, right-click upon it and select
Configure to bring up the configuration options ...
Set the configuration as shown above, click the OK button, and the
application will be minimised; it will return to the System Tray and run in the
background. That's all there is to it.
Configuring the Bush Internet Set-Top Box
The instructions provided here are for the Bush IBX-100 but should be applicable
to all versions manufactured, including those produced for OEM's such as
Freeserve, Virgin and Nationwide Building Society. It is assumed that you are
familiar with using the remote control or remote keyboard, entering and
selecting options; if not, a read of the user manual or a bit of guesswork goes
a long
way.
Connect your Bush IBX STB to your TV, turn it on, and press the yellow
"Internet" button on the remote control. The opening navigation screen should
appear showing "World Wide Web" and "Options" images. Use the remote control
cursor keys to move to select the "Options" graphic ...
Press the "Select" button, and the option menus will appear. Use the cursor keys
to move to the "Telephone" image. Press the "Select" button and the telephone
dialling options will appear ...
Use the remote control cursor keys to select the options required ...
Tone or Pulse Dialing : Tone
Use Dialing Prefix : No
Wait for Dial Tone : No
When the options have been selected, use the cursor keys to move to the
"Complete" image, and press the "Select" button again. This will return you
to the options menu again.
Use the cursor keys to move to the "Timeouts" image. Press the "Select" button
and the timeout options will appear ...
Use the cursor keys to select the timeout required. "Off" will leave the STB
on-line until instructed to turn off and is the preferred option. The others
will set an idle time after which the STB will automatically hang-up its
modem. While important when paying for outgoing calls, a timeout need not be
specified unless you want to.
When the desired timeout has been selected, move to the "Complete" image, and
press the "Select" button again. This will return you to the options menu again.
Move to the "Complete" image if not automatically selected, and press the
"Select" button again. You will be returned to the main navigation screen.
Now things get a little awkward; we have to access the secret ISP configuration
menus.
Access to the secret menu is through the use of a code sequence which is
entered when the main navigation screen is displayed. Although the access code
changed at times, some of the control codes which have been identified, are
in common use, and have been reported to work by a large number of people are as
follows ...
IBX-100 / IBX-101 IBX-200 IBX-250 IBX-260 ITV8401
39d8sk43k2 fk39x39662 4927dkan93 12603belt 86ilm37924
25fh5n66as
81fhm29421
86ilm37924
muf1312129
If none of the codes appear to work, you will have to try the more complex
mechanism of gaining access to the Service Provider configuration menus
described on the Bush IBX
page.Once the correct code has been entered, the Service Provider Screen will be
displayed ...
Select the "Change Details" image and press the "Select" button. This brings
up the first of three ISP configuration screens ...
Set the entries as shown below but for the "Home URL" enter the numbers which
were shown by the SimpleServer:WWW earlier. Include the "http://" prefix, but
do not include a trailing forward-slash. The "0800" telephone number is set to
give a delay when the STB makes its out going call and to make it safe should
it be connected to an external telephone line by mistake.
ISP Name : bushstb
ISP Telephone Number : 0800
User name : bushstb
Password : bushstb
Home URL : http://10.0.0.10 - See above text !
The "Password" will be masked on entry, so take care when entering it, and
count that the correct number of characters have been typed.
When the required details have been entered, move to "Next" and press the
"Select" button. This will bring up the second ISP configuration screen ...
The first three settings are not particularly important for basic web browsing
but may cause problems with the STB software if not provided, and the two DNS
settings may be important if you are using a 'proxy server' to bridge to the
internet through the PC; you will have to determine what these should be for
yourself.
The "Dial script" must not be changed unless you know what you are doing, and
even then it should not be necessary to change it. My STB was previously used
on an NTL service and has the following settings, which should be usable by
anyone ...
ISP domain name : ntlworld.com
Primary DNS : 194.168.4.100
Secondary DNS : 194.168.8.100
Dial Script : "\nATZ\n" "OK\nAT&C1&D ... NECT\n\n" "\n\n"
When any changes required have been made, move to "Next" and press the "Select"
button. This will bring up the final ISP configuration screen ...
For basic web browsing all entries on this screen should be deleted except
the entry for the HTTP proxy server, which, as with "Home URL" above, needs the
numbers which were shown by the SimpleServer:WWW entering. This time, just
include the numbers, no "http://" prefix, and again, do not include a trailing
forward-slash, but add ":6588" at the end ...
E-mail URL :
Search URL :
Time server :
HTTP proxy server : 10.0.0.10:6588 - See text above !
FTP proxy server :
With the entire configuration completed, move to the "Confirm" image and press
the "Select" button. The Service Provider configuration ( using your PC as an
ISP ) will be stored in the STB's non-volatile memory so it will be restored
after being powered-off and you will be returned to the main navigation screen.
The Bush STB is ready for connection to the PC server.
Configuring the PC's Modem
Launch Control Panel, click Modems, then click the Add
button to start the installation of a modem which will communicate at the same
baud rate as the PICAXE will be using.
Tick the Don't detect my modem; I will select it from a list checkbox
and click the Next button. Select Standard 9600 bps Modem
regardless of what modem you actually have, and click Next. Select the
serial port to which the modem is connected and click Next again. When
Windows has installed the modem, click Finish.
When back at the Modem Properties page, select the Diagnostics tab,
select the Standard 9600 bps Modem, and click the More Info
button. After a short while, information relating to the modem should be
displayed; this indicates the modem is working correctly and Windows has it
properly installed. Click the OK button, then Close the Modem
Properties window and Control Panel.
Installing the Dial-Up Networking Server
Use the Start Menu and through Programs, Accessories and
Communications launch the Dial-Up Networking application.
Click the Connections menu, and if the entry at the bottom of the
drop-down menu list is "Dial-Up Server" then the Dial-Up Networking Server is
already installed.
If Dial-Up Networking Server isn't installed, then launch Control Panel,
click Add/Remove Programs, and then select the Windows Setup tab.
Double left-click the Communications option, tick the Dial-Up
Server checkbox and click the OK button. Click the OK button
of the Add/Remove Programs Properties window and the Dial-Up Networking Server
will be installed.
Configuring the Dial-Up Networking Server
Use the Start Menu and through Programs, Accessories and
Communications launch the Dial-Up Networking application.
Through the Connections menu option select Dial-Up Server.
When the dial-up server screen appears, the first thing to do is to select the
tab for the Standard 9600 bps Modem, then select the Allow Caller
Access option ...
Next the password must be set. Click the Change Password button, leave
the Old Password field blank, and enter bushstb in the New
Password and Confirm Password fields, then click OK.
Click on Server Type to configure the link protocol ...
The PPP: Internet, Windows NT Server, Windows 98 option should be
selected, the Software Compression checkbox should be ticked and the
Require Encrypted Password checkbox unchecked.
When configured, click OK, and OK again to place the DUN Server
into the System Tray where it will run in the background.
Making It All Work
Connect the Bush STB to one of the telephone line sockets, and the modem to the
other; it doesn't matter which goes to which. Plug the modem into the Exchange
Simulator, and also connect that to the PC. Power-up the STB ( and of course
your television ), the modem and the Exchange Simulator.
Start the SimpleServer:WWW web server running on your PC - make sure you press
the Start button and check that you can access it through your PC web
browser, and minimise it to the System Tray.
Launch the Dial-Up Networking Server, select the Standard 9600 bps Modem
and enable incoming calls.
Use your STB's remote control to connect to the World Wide Web, sit back and
watch as it dials out, connects, verifies and delivers a page from your PC ...
The first page to be displayed will be the "Yippee" one delivered by
the local web server, and following the two links should bring up the BBC News
home page ( which cleverly works out we have a "Low Graphics Mode" browser ),
and the Google search site delivered through the Proxy Server ...
The Dreaded Dial-Up Networking Crash
When using Windows 98 SE, there is an annoying tendancy for Dial-Up networking
to crash if it can't establish its link to the modem, with the result that when
continuing to use Dial-Up networking, everything appears to be dead, even though
Windows' reports indicate that things are working as they should be.
The problem often occurs in particular when allowing caller access to the DUN
Server when the modem is not connected or powered off, and when the programming
Molex link on the PICAXE Exchange Simulator is in the Download rather than Run
position.
The problem usually identifies itself with a pop-up indicating that Dial-Up
Networking was unable to connect to the modem or the modem failed to respond
properly ...
It would be expected that acknowledging the message and closing the DUN Server
down when using that would be enough to put Dial-Up Networking into a state
where further attempts to use it would be successful once the underlying
problem is rectified, but not necessarily. Further attempts to use Dial-Up
Networking may report successful operation when the modem is unconnected or
even powered off. Needless to say, it can be very frustrating to tell what's
going on if you are imagining that Dial-Up Networking is working properly and
doing its job.
If, after getting a Dial-Up Networking error, closing the DUN Server Down if
using that, you press Ctrl-Alt-Del once ( twice and it will
re-boot your machine, losing all data you've been working with ! ), you should
see the Task Manager window appear ...
If "Rnaapp" is lurking in the task list, then Dial-Up Networking has most likely
crashed. The "Rnaapp" task can be forced to end, but it will not necessarily
resolve the problem. There is often no alternative but to re-boot the PC, while
singing the praises of Microsoft's software.
Project Journal
As indicated at the start of this document, this is a work in progress, and the
following is a brief 'journal' on the development of the Exchange Simulator
development. If you're under the age of 30, you'll perhaps prefer to think of
it as a "Blog".
8th July 2004
An idea which has been floating around in my head for a long while ( over two
years ) suddenly finds the impetus to flower into reality.
Day one is spent tracking down all my notes on telephone systems and getting up
to date with information relating to telephone interfacing on the internet. The
Bi-Directional Telephone Line Simulator
remains as probably the best telephone exchange / simulator projects I have
seen on the internet and is a highly recommended resource for anyone embarking
upon such a project. A more recent project from
Silicon Chip Magazine is a
PICAXE Controlled Telephone
which works along similar grounds.
Despite an intensive search, not one web page or datasheet I could find
suggests or supports my idea that it is possible to build a voice path using
two ( or more ) telephones in parallel with a single resistor pull-up; all use
either capacitor or transformer coupled interfaces, or have the two telephones
in series.
9th July 2004
The day is spent producing the first pass of this web page describing
the project, creating circuit diagrams and working out pin connections for the
various sockets.
10th July 2004
A day off, doing things other than electronics. That does not mean that project
thoughts are not floating around in the back of my mind; the concerns I have as
to whether my proposed design will work, or be a catastrophic failure, are not
easy to allay, as, although a software engineer by profession and familiar with
digital electronics, I'm virtually an amateur when it comes to analogue
electronic design.
11th July 2004
It is disconcerting and nagging at me that no one else has used such a
simplistic system to link two telephones together; either it's been tried and it
didn't work, and was therefore not worth documenting, or no one has ever thought
of trying it, and have relied upon existing designs using the telephones in
series.
Armed with a variable voltage power supply, a dual telephone line socket
connector, a 330R resistor and two analogue telephones, it's time to find out
if my idea works or not.
A single analogue phone works okay; noise picked up by the mouthpiece can be
heard in the earpiece, and dialling DTMF tones can be heard clearly. The setup
draws about 10mA at 9V when off-hook, nothing measurable when on-hook. A meter
across the phone shows 9V when on-hook, dropping to about 6V when off-hook. Two
phones together work just as well; speech from one being heard in the other and
dial tones passing over clearly. The current rises to around 13mA when both are
off-hook.
Time to step into dangerous territory and put 'expensive hardware' into the
circuit. A Bush Internet Set-Top Box is connected to
the Exchange Simulator in parallel with a phone. When the STB dials-out, the
modem signals can be clearly heard. When again connected to a real telephone
line, the STB continues to work as expected, so no apparent damage done.
The phone is replaced by a PC modem; the real 'fingers crossed' test, as
anything amiss could potentially destroy the modem. A dial-out from the STB is
initiated while Windows Terminal is used to talk directly to the modem. Issuing
an "ATA" command causes the PC's modem to pick-up the incoming call, and the
familiar inter-modem whistling and wailing is heard. The elastic band sounding
'boing' encouragingly confirms that high-speed communications are being
established and soon the PPP link establishment packets sent from the STB
appear on the Terminal display - Success !
Various trials were attempted in an effort to use 5V for the telephone line
supply, but with no joy at all below 7V or so, even with a reduced resistor
value; the two modems seemed to get themselves into a tangle with much annoying
wailing from them, and gnashing of teeth elsewhere.
It's disappointing that a single 5V supply can't be used, but it's really a
minor inconvenience with little cost impact. The big news is that the one
resistor pull-up idea works, or at least does for me.
12th July 2004 - Early Morning
Can a Windows Dial-Up Networking Server ( DUN Server ) be fooled into responding
to a "RING" message that's faked to appear as if it's coming from a modem ? It
is fundamental to the project design but it's an untested idea. Time to find
out.
The easiest way to try it is to wire the PC's two serial ports back-to-back,
start up the DUN Server and use Windows Terminal to send back
"OK" replies to any commands sent to the modem, and when that's all done, send
the magical "RING" message and see if an "ATA" ( Answer ) command gets sent
out. Running two copies of Windows Terminal should be enough to check the
loop-back works in both directions before firing up the DUN Server.
The idea was fine but the reality wasn't; whatever Windows Terminal does, it
didn't work on my system when the DUN Server ran, but resorting to writing an
MS-DOS Modem Emulator ( see Appendix 3 )
showed the project's principle to be solid and revealed that DUN Server does
work as expected, although it doesn't seem to care about Carriage Returns after
the "RING" message.
One thing the testing did show was that when the DUN Server was instructed to
use a Standard 2400 bps Modem, it still insisted on communicating with
it at 9600 baud. This appears to mean that the PICAXE-08 is unusable with the
configuration as its maximum serial transmission speed is limited to 2400 baud.
12th July 2004 - After Some Sleep, The Afternoon Arrives
The joys of writing configuration instructions are upon me. It is only when
you start to write instructions on how to do something which is second nature
because of previous experience that you realise what a steep learning curve
there is to becoming truly computer literate.
Although those of you reading this page probably have a good idea of what you
want to do with the configuration described here, it is guaranteed that many of
you have never tried to do anything like this before, and aren't sure where to
start. Trying to guess at reader's technical capabilities and prior experience
is incredibly difficult, and the best I can do is offer some comprehensive
instructions on how it should all happen and hope it goes as well for you as it
did for me. There are far too many possible problems along the way to list and
account for - typing the wrong IP address into my browser kept be mystified for
an hour until I twigged what the stupid mistake was - but hopefully the
instructions will be good enough for the great majority of people who want to
give the project a spin.
Even with enthusiasm, there's only so much writing anyone can do at a time, and
especially so when there are electronic designs to play with; testing with a
real modem beckons.
With a PICAXE development board loaded with an 18X and a serial Y-lead built
with the diode mixing required for the transmit to PC line it's time to see if
the project works with everything connected and the PICAXE generating the "RING"
message with a push of a button.
And yes it does - and brilliantly so too. The Bush STB is instructed to fetch
a page from the PC's local IP address, it takes its modem off-hook, the
PICAXE button is pushed, the "RING" message is sent, the DUN Server springs
into action, answers the incoming call and establishes a connection, and to
great relief the expected page is displayed on my television.
13th July 2004
After a long but fruitless task of trying to make the DUN Server actually
communicate with a 2400 bps modem at 2400 baud ( to enable the use of
a PICAXE-08 ), it's back to the grind of documentation and other distractions,
such as tidying up the house; it's amazing just how much mess 'Rapid
Development' can create, with bits of cut wire and potential components
accumulating in many nooks and crannies.
With tidying up out of the way, it's on to the tortuous task of creating a
veroboard ( stripboard ) layout and track cutting diagram, which can then be
turned into a graphical image to be published as part of the documentation.
The task is invariably harder than imagined, with blind alleys, mistakes and
a great deal of tweaking involved. Like a great work of art, it's sometimes
hard to tell when you've got a satisfactory result, and say "it's done". With
the circuit compressed into just over 1" x 2" without any 'dodgy dealings' it
has been quite a success, and the effort put in has been well rewarded.
With a veroboard design complete though, it means realistic and representative
hardware can be constructed, and any circuit and veroboard layout and tracking
errors identified.
15th July 2004
It's been ages since I've reconfigured a Bush STB ( mine has been running
through a genuine private telephone exchange for over two years ), so time to
get the notes out and wire it all up for video capture to get some screen
shots to explain it properly.
It also seemed sensible to get some 'web proxy' software running and the STB
configured to use it at the same time.
That was fun, more involved than expected, and revealed a tendency for the
STB to hang up its modem while waiting for pages to be delivered through the
proxy. I suspected a problem with the proxy of firewall configuration, but
bumping the line voltage up to 12V seems to have cured the problem; the
connection stayed up for a number of hours and hadn't failed when I
disconnected it by choice.
16th July 2004 - Early Morning
A nightmare of a morning, the sort which makes you wish you'd gone to bed before
doing "just one more thing".
What's the worse thing that can happen when you've been running your modem
through potentially dodgy electronics ? You find you can't dial out to the
internet and use it to upload web pages.
The "Not Responding" messages pile-up as soon as the modem goes off-hook.
Sometimes it dies immediately, sometimes it connects to the ISP only to have
the username or password rejected, and all accompanied by an unuseful
error message; "The other computer disconnected you. Double-click
to try again", and everything's compounded by the
dreaded Dial-Up Networking crash,
with the Dial-Up Networking 'Rnaapp' application lurking rogue-like in the
background having failed to terminate itself appearing to interfere with
further attempts to dial-out.
The first thought, that I'd burnt the modem out, turned out to be unfounded
after a bit of experimentation - it all works fine with the Y-lead interface
being used for the Exchange Simulator unplugged. So there are one of two
possibilities; there's a short or break in the lead, or it doesn't like the
high speed ( 57600 baud ) being used to talk to the modem when the Y-lead is
in place.
My money's on the latter, as it still works fine as a DUN Server at 9600 baud
using the Standard 9600 bps Modem, when a lead fault would also be
expected to generate an error.
Forcing the modem to only use 9600 baud and it's absolutely perfect; panic over,
no damage done beyond paying connection charges for abortive calls to various
ISP's. But it does pose interesting questions as to; why ( I guess it's the
diodes leaving the serial line floating, ringing, or slugging the signal,
but I don't own a 'scope to look at what is actually happening ), what, if
anything to do about it, and what the baud rate limitations are now and with
any potential fixes in place.
Time for further investigations later in the day - hopefully something as
simple as a pull-down resistor from the diode-mixing point will do the job -
the weird thing is, I am convinced I tested this early on in the project's
development.
16th July 2004 - Daylight
Baud rate it is. The modem works fine at up to 38400 baud, but the jump up to
57600 is just too much. As hoped for, a 22K pull-down to 0V solves the problem,
at least for 57600 baud, and hopefully beyond.
Thinking back, I did test the serial link with the Y-lead in place, but with
the modem connected as the "Standard 9600 bps Modem" - ce'st la vie !
Time for an update to the circuit diagram, layout and a trip to the component
supplier to buy the bits of hardware I don't have.
17th July 2004
I now have all bit one component ready to build a prototype. Unfortunately the
missing component is the 4V7 zener diode to protect the PICAXE from any
excessive line voltages.
Although the design should work safely without the zener, it's better to be
safe than sorry, especially as the current test bed is a rat's nest of wires.
This will introduce a short delay to the proceedings while I obtain a
suitable zener, but it will give me a chance to look at some of the other
projects I'm working on.
18th July 2004
One prototype built, and almost working too.
Having a zener in my hand proved to be a bad move. Whatever the problem, the
4V7 zener seemed to be clamping the voltage into the PICAXE at less than 3V.
Checking resistance values meant having to unsolder some components, and by
the time I'd re-assembled the board, I'd lost the zener, somewhere on the
floor - I knew I should have bought two. The voltages present are within
specification anyway, so it's now removed from the circuit and layout.
One thing that clouded my vision by developing with a PICAXE-18X instead of the
target 08M is the maximum serial baud rates allowed. The 18X allows user serial
rates of up to 9600 baud, but the 08M using SEROUT is limited to 2400 baud at
4MHz, 4800 baud at 8MHz; the only 9600 baud communications being serial output
using SERTXD. The original choice to use the Serial Out ( leg 7 ) and SERTXD
probably reflected this. Unfortunately, this means the diagnostics port can only
be run at 4800 baud, not the 9600 originally envisaged, but it is not a major
problem for this project.
19th July 2004
With the prototype complete and tested, it's time to put everything together
and make sure it works in anger.
And it does. A delay between detecting the dial-out modem going off-hook and
the "RING" message being sent was added to stop the two modems getting
themselves into a tangle and stuck in a symphony of cacophony as they
negotiated the connection, and the on-hook and off-hook transition state code
was improved upon to create a much more robust solution and to provide better
Diagnostic Port messages.
Although only tested through a 15V regulated supply and not a mains transformer
or wall-PSU, it all works fine. Any problems with an unregulated supply will
probably be due to ripple, which should be easily fixable.
20th July 2004
Revolution Education Limited announce the launch of the PICAXE-08M ahead of
schedule. It'll soon be time to put one in the prototype and check it works
okay.
Future Plans
The Exchange Simulator has to be tested with a PICAXE-08M when I get one in
my grubby mits, but is not envisaged that there will be any changes required, so
the only things left now, is to test the Exchange Simulator on other systems,
complete the documentation and get someone to work through it to check its
accuracy, and give further examples of using the Exchange Simulator -
configuring a Dreamcast web browser, and getting it to download games and
mini-applications into a Dreamcast's Visual Memory Unit is one particular area
I'd like to cover.
I wouldn't like to put a timescale on when any of this is likely to get done
though !
Appendix 2 - Project History
This is a brief list of all the changes and modifications which have had to be
made during the project development. It should hopefully help anyone else who
wishes to develop their own version of the Exchange Simulator; it's always
handy to know what pitfalls may await and have potential solutions to hand.
The Line Voltage was increased from an initial +9V to +12V
With only 9V, the modem had a tendancy to drop out after only a short period of
connection.
A 22K pull-down was added to the diode-mixed serial to the PC
Without the 22K pull-down, the modem and PC failed to communicate reliably above
38400 baud.
Appendix 3 - MS-DOS Modem Emulator Software
The following program connects its serial port to the serial port a modem would
normally be connected to and is used to emulate that modem and display data that
is received. It is used to see what commands are sent when Dial-Up Networking
Server is activated.
The program reads commands sent to the modem using "AT" commands and replies
with "OK" to each. Pressing the "R" key sends a "RING" message out, and
pressing "Q" terminates the program. The program is written in FirstBasic /
PowerBasic.
PORT$ = "COM2:9600,N,8,1"
OPEN PORT$+",RS,DS" AS #1
ON ERROR GOTO TRAP
DO
WHILE LOC(1) <> 0
C$ = INPUT$(1,1)
IF C$ = CHR$(&hD) THEN
PRINT
PRINT#1, "OK"+CHR$(&hD);
ELSE
PRINT C$;
END IF
WEND
K$ = UCASE$(INKEY$)
IF K$ = "R" THEN
PRINT#1, "RING"+CHR$(&h0D);
END IF
LOOP UNTIL K$ = "Q"
CLOSE#1
SYSTEM
TRAP:
RESUME NEXT
The following interaction is observed by the modem emulator ( "OK" responses
and "RING" message not shown ) ...
Caller access allowed ...
AT
ATE0V1
AT
ATS0=0
DUN Server waits for "RING" message ...
The "RING" message is sent ...
DUN Server instructs modem to answer the incoming call ...
ATA
PPP link establishment packet data sent from DUN Server
PPP link establishment fails ...
AT
ATE0V1
ATH
ATZ
AT
ATE0V1
AT
ATS0=0
DUN Server waits for "RING" message ...
Incoming calls disallowed ...
ATZ
The "AT" commands are presumed to be there to force the modem to the correct
baud rate ( modems use auto-baud rate sensing ) and to illicit "OK" messages
from the modem to confirm its correct operation. The "ATS0=0" command instructs
the modem to issue a "RING" message when the first ringing signal is received.
The "ATA" command instructs the modem to answer the incoming call. The "ATH"
instructs the modem to hang-up the call. "ATZ" and "ATE0V1" are used to set the
modem into the mode required by the DUN Server; Factory Default, No Echoing of
Commands Sent, and Low Speaker Volume.
Appendix 4 - Establishing Line Level Settings
Line Level Settings ( the values required for the MIN_ON_HOOK_LINE_LEVEL and
MAX_OFF_HOOK_LINE_LEVEL constants in the PICAXELS.BAS program presented
earlier ) can be determined by monitoring the diagnostic output from the
program. This is a serial transmission at 4800 baud through the Diagnostics
Port if fitted, or from Leg 3 of the PICAXE.
The Telephone Exchange Simulator program ( PICAXELS.BAS ) should be downloaded
and then powered-up with two analogue telephones connected with either
both on-hook or with just one off-hook.
Ignoring any "On-Hook" and "Off-Hook" messages, the line voltage data is
returned as two decimal numbers; the minimum and maximum values read from the
telephone line socket line since started, so the program needs to be
run twice for both on-hook and off-hook situations. To restart the program, turn
the power off and then back on.
Run the program for a short while in both situations, and record the range of
readings obtained. Talk and shout down the phone when off-hook, and press the
dialling keypad to ensure the voltage changes caused are noted, although they
may well be too small to register on some phones. Note that the levels are only
transmitted when a change in reading is detected.
Take the lowest reading obtained when on-hook, this is the
MIN_ON_HOOK_LINE_LEVEL and take the highest reading obtained when off-hook, this
is the MAX_OFF_HOOK_LINE_LEVEL.
These two values should be entered into PICAXELS.BAS program using the
Programming Editor and the new program downloaded into the PICAXE.
The program itself will determine the gap between these two readings and will
create two offset points from the on-hook and off-hoook values to create two
points which are then used to determine if the modem is on-hook or off-hook ...
MIN_ON_HOOK_LINE_LEVEL - MAX_OFF_HOOK_LINE_LEVEL
LINE_LEVEL_OFFSET = ------------------------------------------------
3
ON_HOOK_LEVEL = MIN_ON_HOOK_LINE_LEVEL - LINE_LEVEL_OFFSET
OFF_HOOK_LEVEL = MAX_OFF_HOOK_LEVEL + LINE_LEVEL_OFFSET
The Dial-Out Device modem will be deemed to be on-hook when the voltage read is
greater than the ON_HOOK_LINE_LEVEL and off-hook when lower than the
OFF_HOOK_LINE_LEVEL - Note that the voltage drops when going from on-hook to
off-hook.
The software continually tracks the line voltage so any small 'glitches' which
make the modem appear on-hook when it is off-hook, and vice-versa, are ignored.
Line Voltage Readings
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The image shows the voltages read at the Line Socket when the phone
is on-hook ( left-hand side ) and when off-hook ( right-hand side ). The two
grey bands show the range of voltages read in both cases, and the upper blue
line marks the determined ON_HOOK_LINE_LEVEL, and the lower green line marks the
determined OFF_HOOK_LINE_LEVEL. When the voltage is continuously above the blue
line the phone is deemed to be on-hook, and off-hook when below the green line.
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PICAXE is a trademark of Revolution Education Ltd.
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