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!
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.
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...
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..
and did they have standard connector?
No
I can see 2 wire pairs, and ground.
No ground. Some loops are single-ended, so on-off keying, but in the UK
we used double ended, so reverse current direction.
Dave
