[cctalk] Re: RS232 then and now

Low speed modems are just analog devices that can pass any signal up to whatever the design speed limit is. For example, a 103 modem is good up to 300 bps, but will happily carry anything less. A 202 modem (see below) is designed for 1200 but also will work at lower speeds, and has been used at 1260 bps. Faster modems use signalling schemes that involve clocked signal elements. QPSK (for the 212 modem) is likely one of these, and for the speeds above that it is definitely true. So there the modem isn't just an on/off signaling pass-through device. More accurately, for half duplex line control, as others have pointed out. On a half duplex circuit the device (DTE) would assert RTS when it wanted to send, and when the modem had decided to turn the line in that direction it would signal CTS to indicate the device can send now. If the device was done, it would drop RTS; alternatively (I suppose) the modem could drop CTS on its own initiative to rescind permission to use the line. I once worked on devices that used asynchronous communication, half duplex, multipoint (Harris 2200 display advertising editing terminals, used by newspaper). That was done with DL-11/E devices, and the modem signaling I just described did the direction control. It even worked most of the time, though I'm quite happy to have dealt with those contraptions only once. Or for split-speed communication. Yes, by the time you have all that you're close to using up 25 pins. Another consideration is that equipment at the time was much larger than nowadays so the connector used was actually quite small for the time. Some other modems used the V.35 standard (for high speeds) with a connector whose designation I don't remember that's a whole lot bigger than the DB25. An example of that is the not so well known Bell 202 modem. That uses FSK signaling like the 103 modem but at higher speed, with a shift of (I think) 1000 Hz. Between that and the data rate it ties up the entire voice channel one way, so if you wanted to use it in full duplex mode you'd need four wires, rather than the standard Telco 2 wire circuit. Alternatively you could use two wires half duplex. An example of the latter is the original Ham radio "packet radio" system, which used 202 signaling carrying HDLC frames. A standard 202 modem would do that just fine since it's a purely analog device, but you need a separate clock recovery circuit. Another place where 202 signaling is found is in the original PLATO IV terminal commmications system. That is 1200 bps (or more precisely 1260 which works too) outbound to the terminal, and something slow (126 bps) the other way. When carried over a modem that would use the "1200" baud regular channel of the 202 outbound, and the secondary channel inbound. There's enough bandwidth in the phone line to tuck away an FSK band slice for that slow speed in what's left over from the 1200 bps channel the other way. I think polar relays go all the way back to telegraph line practice, perhaps because they improve noise immunity. The current loop serial devices I have used are all on/off keying, though. Those were ASCII (Teletype model 33 and some early 300 bps terminals) which use 20 mA loops. Earlier Baudot devices like the Model 19 use 60 mA loops, at data rates around 45 bps with 5-bit coding. I assume newspaper wire service lines (6 bit code) were current loop too but I don't think I ever saw the details. FWIW, another place where you find current loop systems is in industrial control and sensing systems. Those tend to be analog, described as "4-20 mA loop" -- for example, a temperature sensor can do this, encoding whatever its temperature range is into a current between 4 and 20 mA. The idea behind current loops is that mechanical teleprinters are severely inductive loads -- basically you're driving an electromagnet. To get decent waveforms the system impedance should be mostly resistive (i.e., moderate phase shift). So you construct the circuit with a large series resistor and a supply voltage chosen to get the current you want. And yes, the other advantage is that you can go farther by increasing the drive voltage, if the distance is large enough that wire resistance, or circuit inductance, is significant. I remember our college timesharing system using current loop terminal runs halfway across campus, well over 1000 feet. One of those was for a Beehive editing terminal which was certainly 300 bps and maybe faster. paul