[cctalk] Re: RS232 then and now

On 31/01/2025 08:20, Steve Lewis via cctalk wrote: Almost forgot - why 300 baud? The modem the RS232 was driving was modulating over standard telephone lines, which are obviously audio.The original ones used Frequency Shift Keying (FSK)- basically switching between high and low tones for zero and one. You need a certain amount of tone to be present for a bit to register. Given you want full duplex you need four frequencies (two each for high and low). These were 1080Hz, 980Hz, 1750Hz and 1650Hz - i.e. 100Hz apart for two pairs. The pairs needed a gap of around 600Hz between them so they wouldn't be confused after the telephone system had mangled the sound. In Europe the POTS bandwidth was around 3K, in the USA it was 2K (but not starting at zero) so these frequencies were sufficiently inside the available bandwidth to get to the other side. If you up the bps you come to a point where there won't be enough of the mark frequency to be detected - it won't have enough complete cycles. And that point was basically 300 baud. 980Hz divided by 300 gives about 3 complete cycles to indicate a mark, which is the minimum you need for easy detection by an analogue circuit. Incidentally, the US had Bell 103 at up to 300 baud, Europe and V.21 which was slightly better but used the same frequencies. When originally launched they were used at 110. You also get problems squirting tones through the telephone system it also uses tones for internal signalling and if you send harmonics you'll re-route the call. Bell 212 at 600/1200bps (aka V.22) used more complex QPSK modulation to encode more than one bit per cycle. That's the one that sounded like someone being sick down the phone line, as one of my colleagues put it. Eventually more complex standards got to a total data rate of 56Kbps. Given the telephone network by this time digitised at 64kbps, you'd be breaking the laws of physics if you got a higher rate than that. Getting 56K was pretty impressive in the circumstances. Regards, Frank.