How much of the 8085's time will be left to do anything useful at all with the data with it essentially bit-banging the waveforms like that?
On HF, packet radio is 300 baud using Bell 103 tones already. I've done some reading over the schematics and it looks like the onboard modem can be put into a test mode (pin 2 high) where it uses the same tone pair for RX and TX. The analog filters can be similarly aligned by lifting one trace from the ANS/ORG switch. The frequencies of the tones themselves aren't a problem since you can just offset the tuning on your SSB transceiver to compensate. With the modem and UART hardware doing the hard layer 1 work, the CPU should have plenty of cycles to spare to deal with the bit stuffing, encoding, CRC checks, AX25 packet structure, etc. That and it would just be cool to have a cable going out the modem jack straight to a transceiver and log on to some BBS on battery power. On Fri, Apr 16, 2021 at 1:06 AM Douglas Quagliana <dquagli...@gmail.com> wrote: > 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 >> > -- Disclaimer: Any resemblance between the above views and those of my employer, my terminal, or the view out my window are purely coincidental. Any resemblance between the above and my own views is non-deterministic. The question of the existence of views in the absence of anyone to hold them is left as an exercise for the reader. The question of the existence of the reader is left as an exercise for the second god coefficient. (A discussion of non-orthogonal, non-integral polytheism is beyond the scope of this article.) Thanks /usr/games/fortune
