Question about modems
bhilpert at shaw.ca
Wed Nov 13 20:34:12 CST 2019
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
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.)
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