31 Jan
2025
31 Jan
'25
4:30 a.m.
On 31/01/2025 11:56, ben via cctalk wrote:
On 2025-01-31 4:25 a.m., Frank Leonhardt via cctalk
wrote:
Just noticed in the bit you quoted - "RS232 is not serial" - that could
be taken the wrong way!
The 25-pin D-sub was a recommendation, not a requirement (at least
initially). There were 11 pins in the standard but sometimes other pins
were used for a synchronous clock and vendor specific stuff.
I really keep banging on about it, but RS232 was for controlling a modem
and most of the pins are to do with that. RS232 has come to be used to
mean (incorrectly) asynchronous serial in general, which ain't helpful.
So current loop - it's the same idea as your (analogue) telephone line,
and it can be used to transmit over long distances as the current is
constant. You can push up the voltage at the start to overcome loop
resistance until the current reaches the value you need.
And for current loop you need two "pins" - loop in and loop out. It's
just transferring a signal; it doesn't need extra pins for controlling a
modem or whatever. You send a mark (1) by breaking the loop - the
current stops flowing at all points 'cos that the laws of physics.
If you think about it, you can have as many listening devices on a
current loop as you like. Each will see the character bits in the form
of the current coming off and on, and each will print the same. This was
used with IBM DCCUs and suchlike to have identical printouts at
different stations in a bank.
It also means it's half duplex! Although you can have two current loops
(four wires) for a full duplex application.
IIRC RS-232 was only specified to work over 50 feet (15m) at maximum
data rate 19,200. Whether it'd work at higher speeds over that length is
questionable. Current loop circuits for teletypes would work over 500m,
no problem at all. As long as the current source could have the voltage
tweaked up high enough to overcome resistance on the loop. There did
come a point where the voltage needed would be too high for safety at
the near end, of course.
Regards, Frank.
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!
I always wondered why one needed a 25 pin connector?
Now every thing seems to be just 3 wire TTL.
Before RS232, how many wires where needed for the current loop
and did they have standard connector?
I can see 2 wire pairs, and ground.