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HOW TO CONNECT A CELLFONE TO A MODEM
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MODEM
T1 | | T2
pin 1 o--||--o___ ______o o______ ___o--||--o pin 6
|?| |?|
|?| 1b 2b |?|
|?|___________________|?|
1a |? _____ ____________ ?| 2a
|?| \/ |?|
|?| 1c /\ 2c |?|
pin 24 o__________|?|___/ \__/\/\/\___|?|_________o pin 24
1K
preset
adjusted to give best
tx/rx isolation
To connect a modem to a motorola 4800.
You need :-
2 off 2.2uF capacitors. (non polarized)
2 off isolation transformers (signal) with 3 seperate windings
1 off 1k ohm preset
1 off BT socket (to plug modem into)
Using a capacitor to provide DC isolation, connect winding (a) of transformer
1 to pin 24 on D plug and the other end of winding (a) to pin 1 of the D plug
via a capacitor.
Connect winding (a) of transformer 2 in a similar manner to pins 24 & 6.
(see above).
Winding 1(b) & 2(b) are connected as in above picture, the start of each winding
goes to the BT socket (pins 2 & 5), and the two ends connect to each other.
Winding 1(c) & 2(c) are connected as in picture, the start of winding 1(c) is
connected to the end of winding 2(c), and 2(c) start is connected via a 1k
preset to the end of 1(c).
The way it werks is thus.
A signal being presented to winding 1(a) will appear on both 1(b) and 1(c).
it will then couple to transformer 2 through 2(b) and 2(c), BUT because
winding (c) is connected out of phase, it will cancel out the signal induced
by winding (b) thus giving no signal at 2(a).
A signal from 2(a) will follow the same proceedure and not appear at 1(a),
but a signal at either 1(a) or 2(a) will be present at the modem. (actually
only 50% of the signal will reach the modem, the other 50% is eaten up by
the balancing resistor.
The signal from the modem is delivered to both 1(a) & 2(a), (50% of signal
is present at both ports), but as pin 1 is the microphone input to the fone
it will accept the signal and send it on its way. Pin 6 on the other hand
is the received audio, and it don't care if there is a signal connected to
it or not as it is an output.
The pin numbers given are for the Motorola 45/48/6800 mobile/transportable,
but the basic idea should werk on any cellfone. It is also a good idea to
disable the microphone and earpiece in the handset once the modem is
connected, otherwise (a) the noise from the earpiece gets on your tits, and
(b) the microphone tends to pick up your 100 gigawatt stereo system and send
the output of same to the remote modem, this causes transmission errors as
remote modems don't appreciate or understand 'techno techno techno', at high
volume levels.
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? WRITTEN AND UPLOADED BY ?
? DaveX ?
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modified version for better cps
MODEM
T1 | | T2
pin 1 o--||-, ,--o___ ______o o______ ___o-, ,---||--o pin 6
| / |?| |?| \ |
| \ |?| 1b 2b |?| / |
|___ / |?|___________________|?| \___|
2.2K preset\ 1a |? _____ ____________ ?| 2a / 2.2K preset
/ |?| \/ |?| \
\ |?| 1c /\ 2c |?| /
pin 24 o____|______|?|___/ \__/\/\/\___|?|_____|____o pin 24
2.2K
preset
adjusted to give best
tx/rx isolation
Set all presets to mid position, LOCK MODEM TO 12000 BAUD or 9600 if you
not got 12,000 and connect to a USR modem somewhere. Keep adjusting both
recieve and transmit presets untill max cps is achieved. (boring, time
consuming, fiddly, a real pain in the arse. but worth it.)
Make sure you set fone volume to max before making adjustments or next time
you try and use it, the settings will be wrong.
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