Serial port connector conventions - Why & When did they change? What's the history?

[Chuck(G)]

Secondary channel is part of V.23, though rarely found on most computer asynchronous modems. Very slow speed (<300 bps), the intention apparently is for modem control, particularly when half-duplex is involved. Few remember that the 202S modem used on voice-grade lines was half-duplex. (The 202T was a 4-wire modem and capable of full-duplex operation).

 

[mdog69]

As I recall the early RS-232 standard had a bunch of "secondary pins" (which I always guessed was for backup/redundancy since lines were noisier or lesser quality back then). But as alternative error correction methods were developed (with faster systems) or more reliable infrastructure, the need for those secondaries became less and less.

Another explanation was that the secondary connections provided a "control channel" in parallel with the main "data channel". For example with a modem you could use the control channel for onhook/dialling etc, and then have the data channel directly connected to the electronics that did the modulation and demodulation. That way you don't need to have anything in hardware to switch the primary serial between "control" and "data".

All this would have been 1970s early 80s, and it's highly likely that the standards that defined the secondary channel arrived too late to be useful to anyone.

 

[Chuck(G)]

As I mentioned, one use of the secondary channel was to interrupt transmission in a half-duplex setup.

 

[stepleton]

Some people would say that RS-232 doesn't have enough extra channels and signal lines, and people like them are the ones who gave us RS-449. A DC-37 wasn't enough, so there's a DE-9 as well: an entire secondary connector!

 

[Chuck(G)]

Some people would say that RS-232 doesn't have enough extra channels and signal lines, and people like them are the ones who gave us RS-449. A DC-37 wasn't enough, so there's a DE-9 as well: an entire secondary connector!

While 442 was used extensively where differential low-voltage signalling was an advantage (e.g. ST506 hard disk interface), 449 was pretty uncommon. Eventually, it was superseded by RS-530 which went back to the 25 pin interface.