Question about modems

[Brent Hilpert]

On 2019-Nov-13, at 5:22 PM, Electronics Plus via cctalk wrote:
> ... I can simply connect 2 modems to each
> other via a POTS phone cable, attach each modem via serial cable to a
> different laptop, and set one to Listen and the other to Answer via
> HyperTerminal.

This will work for some modems, not for others. At the electrical level there were two techniques used for transmitting the signal onto the phone line.

- One technique used a transformer to couple to the phone line, thus the modem actually injected audio-frequency (AF) energy onto the phone-line. The simple back-to-back POTS cable connection may work for modems using this technique. (I have successfully done this.)

- The other technique used opto-isolators to couple to the phone-line. They did not provide energy to the line, they AF-modulated the phone-line central-office-supplied DC current. For modems using this technique, you have to supply a line current. The old 9V battery in series in the loop trick might work but I don't recall whether I've tried it.

(Result of experiments and internal observations with several modems years ago, where I was attempting as you suggest and wondered why it worked with some modems and not others. I think I was trying to do local data transfer with macs that had internal modems but no 232 port.)