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[aprssig] Convert Modem to TNC

Rick Green rtg at aapsc.com
Sat Oct 23 22:23:51 UTC 2004

On Sat, 23 Oct 2004, William McKeehan wrote:

> I have an old 300/1200/4800/9600 modem sitting around the house. It occurs to
> me that a modem and a TNC perform the same basic type of operations. Are these
> two units similar enough that I could convert an old modem to be a dumb TNC?
The short answer: No.

The longer answer:  Modulation and demodulation of the digital signal into
analog tones is only one function of a TNC.  The ax.25 protocol is based
on x.25, which is based on SDLC, which stands for Synchronous Data Link
Control.  The critical term there is 'Synchronous'.  The ubiquitous PC
serial port (now becoming the 'legacy' serial port), is an asynchronous
port.  The support of the timing, clocking, header and CRC generation
requirements of a synchronous datastream is the larger task.  The final
modulation of the signal is trivial by comparison.
  Also, there are several standards for 1200 baud modems.  The one used by
amateur packet is not the one implemented by typical home dial-up modems.
If you happen to have a commercial modem designed for a dedicated
four-wire circuit, it's more likely to be usable for amateur packet.

Rick Green

"They that can give up essential liberty to obtain a little
 temporary safety, deserve neither liberty nor safety."
                                  -Benjamin Franklin

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