Michael Rigby-Jones wrote: [snip] > > > > | < Connect to negative terminal of PS/Battery > > ---------+ | +-------------+ > > modem A |Red -------+-----| modem B | > > |Green -------+---| | > > ---------+ | +-------------+ > > | < Connect to positive terminal of PS/Battery > > > > The power supply MUST be well filtered & regulated. Any power supply hum > > on > > the telephone line will probably ruin any hopes of high speed data > > transmission. > > > > > This surely can not be correct, the presence of a nice low impedance battery > or PSU would effectively short the lines for any signal, making comms > impossible. I thought the the supply was effectively in SERIES with the > line? > In real, both devices have a phone line transformer that connects to the phone line, and that's all. Of course, signals generated by one device is transfered to the other via the transformers, however, the signal's power is very low and it is not enough to make a clear "conversation". What happens in real, is that if you supply a current to the phone line, it will flow through the secondary coil (impedance) of the transformers. When your modem inject a signal at the primary coil, it is changing the transformer coils impedance, so in real it is modulating a bit the current flowing at the secondary. The other transformer is also affected by this current modulation, so the other device "hears" the signal sent by the first device, much better than without this "carrier modulated current". You have two ways to do that: a) You can insert a battery in series with the phone line, it means, one phone line wire will be cut and have a battery inserted closing the circuit. By this way, a closed loop current will be flowing in a circular path through this circuit. There is a problem; It creates echoes and spurious noises because the current is in loop. b) Based on the ASCII design above, you can insert a "ballast" or "pull-up" resistor between the Green wire to the positive of the battery or power supply. This way, the current will be splitted and flow towards both transformers at the same time. The pull-up resistor ensures (Voltage) modulation from one transformer to another. The difference here is that *if* the pull-up resistor is in the middle of the phone line (not close to one transformer or another, as happens in our actual phone switching centrals), the current will flows in opposite directions from the supply point to the transformers, and it is less sensitive to echoes, reflections and EMI interference, and the customer will always know which side is grounded. + 12Vdc | R 270 to 500 Ohms Current R Modem #1 <--- | ---> Modem #2 ---. .-------------o--------------. .--- 3|( 3|( 3|( 600 Ohms 600 Ohms 3|( 3|( Phone Line 3|( ---' '-------------o--------------' '--- | Ground If you want to simulate the phone line voltages and currents: + 48Vdc | R 2 kOhms Current R Modem #1 <--- | ---> Modem #2 | ---. .----RRR------o------RRR----. .--- 3|( 150 Ohms 150 Ohms 3|( 3|( 3|( 3|( Phone Line 3|( ---' '-------------o--------------' '--- 600 Ohms | 600 Ohms Ground If you want to simulate the "Ringing Signal", use this circuit: + 48Vdc | o------>\ <----. | \ | Relay Contact, clicking | \ | at 20 Hz | o GND | | 2 kOhms R === R --- 10 to 30uF | | | | | / | o--o/ o---' | RING SW | Current | Modem #1 <--- | ---> Modem #2 | ---. .----RRR------o------RRR----. .--- 3|( 150 Ohms 150 Ohms 3|( 3|( 3|( 3|( Phone Line 3|( ---' '-------------o--------------' '--- 600 Ohms | 600 Ohms Ground Wagner Lipnharski