On 31/01/2025 13:37, David Wade via cctalk wrote:
On 31/01/2025 11:56, ben via cctalk wrote:
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...
Actually, RTS and CTS weren't used for hardware flow control initially -
think about it - what would they be controlling? The modem was an
analogue circuit and there was no way it could pass flow control down
the line. With smart modems there was buffering involved, and therefore
the need for local flow control other than the XON/XOFF characters which
you wanted to pass through to the remote end.
RTS could be used to seize a half-duplex line (DCE didn't mean full
duplex modem exclusively), and CTS would be asserted once the line was
seized so you could start typing, start the tape reader.
Confusingly both the computer and terminal are called "Data Terminal
Equipment" (DTE) and the modems are data communications equipment or DCE,
Indeed much fun was had with that one. As I said it only makes sense for
attaching something to a modem :-)
Fortunately the main lines come in pairs (DTR/DSR, RTS/CTS, TxD/RxD) so
you can make a mirror cable to connect a terminal to a computer port
that's expecting a modem (aka a null modem cable). If the computer was
expecting a terminal it'd be configured DCE port so the terminal could
plug in using a straight through cable.
But it's not orthogonal. DCD and RI are from DCE to DTE only. As a V100
didn't have a carrier signal and you didn't need to "ring" the computer
it didn't really matter.
Regards, Frank.
