[aprssig] Convert Modem to TNC

William McKeehan mckeehan at mckeehan.homeip.net
Sat Oct 23 17:31:49 CDT 2004

OK, from some of the answers that I have seen I have the following to add
about the modem that I have.

1. It has two modes of operation Synchronous and Asynchronous
2. I have an to mode options SDLC and BSC

But even with that, it sounds like something - if even remotely possible -
would be VERY difficult - I would have a better/easier solution to build a
TinyTrak or TNC-X.

Thanks for all of the responses!

On Sat, October 23, 2004 6:23 pm, Rick Green said:
> 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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William McKeehan
Internet: mckeehan at mckeehan.homeip.net

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