All, I haven't tried the Bell 103 modem, but the cassette port is (in theory) fast enough to see 1200 baud AFSK. The cassette port is supposed to run at 1500 baud. To receive AX.25 packet you would need to count the time between zero crossing similar to the way the cassette port does it now, and figure out if the current bit was a mark or space, then shift the bits into memory as they are received. Upon receiving the trailing HDLC flag you would need to undo the zero bit stuffing, undo the NRZI encoding, and check the CRC. The problem is that Bell212 (1200 baud packet radio) shifts AFSK tones when one bit time has elapsed (1/1200th of a second) and that doesn't exactly match up with the zero crossings for the 2200Hz tone, so you get a whole 1200Hz tone starting at whatever phase the waveform was then at, but more than one 2200Hz tone per bit and the waves are contiguous so the next one just starts where the previous one ends. It doesn't start the phase over at zero for the next bit.
The M100 ROM has code for reading the cassette input pin at 6FDBH and it will watch the cassette port input pin connected to the 8085 SID on pin 5 with the RIM instruction, and then return the number of t-states until the wave will end. This would have to be rewritten to take into account the different frequencies for packet radio versus what the cassette frequencies are. For details on the routine at 6FDBH see https://ftp.whtech.com/club100/ref/rcmap6.100 and go down to 6FDBH. I've already written code for AX25 for PCs that will undo the zero bit stuffing, NRZI and CRC, but didn't get to the bit detection on a M100. I did get as far as a "read the frequency on the cassette port" but not for 1200 baud bits. I think it would be neat to receive and perhaps send packet radio on the cassette port without a TNC. This has been on my bucket list for a long time to see if it could even be done. If you're interested or if you know more about the cassette port routines, then please let me know. Douglas On Wed, Apr 14, 2021 at 10:54 AM Alex ... <abortretryf...@gmail.com> wrote: > Figure this would be a fun one to share with the [M100] list. :) > > I recently bought a big box of random ham radio packet gear which included > a bunch of old TNC modems and assorted cables. Unfortunately, it turns out > the quad serial port card in my desktop PC is dead. > > Enter the Tandy 102 to the rescue! I was able to test all 4 of the TNCs on > the air and sent a test email from the T through a local Winlink RMS node. > > This whole exercise got me wondering if the built-in Bell 103 modem could > be adapted for HF packet radio use. Has anybody tried that yet? > > Pictured in the attached photo is the Tandy 102 hooked to a MFJ 1274 > modem, monitoring Network 105 traffic on 7104khz. > > -Alex >
