[aprssig] Convert Modem to TNC

scott at opentrac.org scott at opentrac.org
Sat Oct 23 17:11:14 CDT 2004

I believe the 1200 baud format used by most telephone modems is Bell 212
phase shift keying, while packet uses Bell 202 frequency shift keying.  The
frequency separation in Bell 103 300-baud FSK mode is much wider than what's
used on 300 baud HF packet.  I do remember reading an OLD Hayes modem manual
(SmartModem 300 maybe?) that had instructions for connecting it to a radio
for < 300 baud teletype modes or something along those lines.

Caller ID uses 1200 baud FSK - I think it's Bell 202, but it might be V.23 -
but all that really gets you is the ability to reuse some chips.  The cheap
desktop caller ID units don't even give you that, most likely - they use a
highly-integrated custom chip that does all the caller ID and user interface
functions and can't really be adapted to any other use.

So the short answer is no, I don't think it'll do you any good.  I've got a
whole drawer of old modems around here someplace, but they're all pretty
much useless.  I just keep them around for parts.. mostly crystals and


-----Original Message-----
From: aprssig-bounces at lists.tapr.org
[mailto:aprssig-bounces at lists.tapr.org]On Behalf Of William McKeehan
Sent: Saturday, October 23, 2004 2:56 PM
To: aprssig at lists.tapr.org
Subject: [aprssig] Convert Modem to TNC

I have an old 300/1200/4800/9600 modem sitting around the house. It occurs
me that a modem and a TNC perform the same basic type of operations. Are
two units similar enough that I could convert an old modem to be a dumb TNC?

William McKeehan
Internet: mckeehan at mckeehan.homeip.net

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