In order to directly connect two modems without a real phone line, one option is to put a battery inline (to create the current loop?). Most information I've found about this relates to the SEGA Dreamcast which has a built in modem and people generally suggest creating something like this:


simulate this circuit – Schematic created using CircuitLab

I don't fully understand how phone lines are expected to behave but this seems incredibly sketchy. I think the 'proper' way to handle this situation is to utilize an FXS Analog Telephone Adapter, but those cost money and I can't find any schematics for how they work. Is there a better way to do this? Or is this approach totally fine?


2 Answers 2


There is no issue with using a battery (or low-noise, isolated power supply) in series with the 600Ω resistor (and capacitor to bypass audio frequencies), as long as the circuits are otherwise isolated (i.e., there is no common ground). After all, the original specifications for POTS (Plain Old telephone Service) was 600Ω impedance (and 48 VDC open-circuit voltage) in most countries, and power was supplied through banks of wet cells (lead acid batteries).

A ringer signal of ~100 VAC, 20 to 40 Hz ("cycles per second" in older documents) was used, and the phone bell had a capacitor to resonate at the particular frequency, so only one phone would ring on a shared "party" line.

If either (or both) of the modems have internal transformers on the phone line, then the resistor/battery/capacitor supply will work, with some caveats.

  • 9 VDC might be insufficient; you might need to snap three or four 9-volt batteries in series. Test and see.
  • There is no ringer voltage, so your modems might need to be manually taken "off hook", i.e., turned on.
  • Batteries will only last a few hours in use.

If there is no transformer isolation, then you'll need a 600Ω:600Ω audio transformer, and batteries on both sides of the transformer.

This worked well to connect older computers, such as two Atari 800 XL's, through 1,200 bps or slower modems. Ah, for the old days of wooden bread boards and bailing-wire connections...

That said, you can buy telephone subscriber-line interfaces, if you have a long-term need.

  • \$\begingroup\$ The resistor in this circuit is just current limiting, I don't think it has to do with the 600Ω impedance right? \$\endgroup\$
    – Ben Longo
    Commented Feb 15, 2021 at 7:35
  • \$\begingroup\$ But I think your right - I need a SLIC. \$\endgroup\$
    – Ben Longo
    Commented Feb 15, 2021 at 7:36
  • \$\begingroup\$ The resistor is needed, both for current limiting and to insure line impedance that would have been provided by loooong copper wire pairs.(though impedance mismatch at audio frequencies, frankly, would have insignificant effect over short lines). \$\endgroup\$ Commented Feb 15, 2021 at 21:43

Your circuit will work but I'd raise the supply voltage to something like 20 volts: -

enter image description here

The DC line detect circuits inside a modem will probably need to see above 5 volts for them to recognize that there is a line connection present. Given that both modems are in series regarding DC, a 20 volt supply would be needed for a comfort factor in this respect.

The 470 nF should be sufficient to put the two devices in parallel regarding AC signals.

I used to design POTs so my advice should be good!


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