> On Jan 31, 2025, at 8:37 AM, David Wade via cctalk <cctalk@classiccmp.org>
> wrote:
>
>
>
> On 31/01/2025 11:56, ben via cctalk wrote:
>> On 2025-01-31 4:25 a.m., Frank Leonhardt via cctalk wrote:
>>
>>> A couple of points you might like to consider, which you may already know
>>> but stuff you've said above doesn't spell it out:
>>>
>>> RS232 is not serial - make yourself clear. Before RS232 the same data
>>> format was used in current loop (often 20mA or 60mA).
>>>
>>> RS232 (AKA V.24) is only understandable when you realise it was connecting
>>> a terminal (or later computer) to a modem. It's very specific, yet like
>>> most technology has been subverted for other purposes. I've kept at last
>>> one full RS232 modem in my loft (it was government surplus, and I used to
>>> to run a BBS in 1980). Things got weird later, particularly with the Hayes
>>> Smartmodem, but modems were dumb devices. The lines went straight through.
>>> There were two oscillators (for FM) and the appropriate one was switched in
>>> by the TX line being high or low. Likewise the data separator looked for a
>>> high or low tone and flipped RX between -12V and +12V. These were all
>>> individual boards!
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.
>>>
>> I always wondered why one needed a 25 pin connector?
>
> Lets look at some of the other pins:-
>
> RTS/CTS - Request to send/clear to send - Hardware flow control.
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.
> DTR/DSR - Is comms up and running
> RI - Ring Indicator - a call has arrived
> TCK/RCK - Used for timing on synchronous links so BI-SYNC/SDLC/HDLC
> LL/RL - enable loop back...
>
> There is also a secondary channel which can be used for out-of-band control...
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.
>> Now every thing seems to be just 3 wire TTL.
>
> Lots of thinks need hardware handshake...
>
>> Before RS232, how many wires where needed for the current loop
>
> So RS232 was designed for modems, so audio tones on a phone line. So:-
>
> Computer <= RS232=> Modem <= audio on phone line => Modem <=RS232=> Terminal.
>
> Confusingly both the computer and terminal are called "Data Terminal
> Equipment" (DTE) and the modems are data communications equipment or DCE,
>
> Current Loop was a physical connection, no modem, so just 2-wires for
> half-duplex, 4-wires for full duplex. For long lines you can use polar
> relays..
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
