15 Dec
2008
15 Dec
'08
5:30 p.m.
[RS232 cables]
I would agree, but then the RS232 signals would need a re-think. They are
not totally symmetircal. For example, DCD (Carrier Detect) and RI (Ring
Indicator) are outptus on the DCE, inputs on the DTE, and there's no
obvious signal that does the same thing going the other way. That's why
many null-modem cables are not simply pin-swaps (there are jumpers
between pins as well, for example crossing STR with DSR-strapped-to-DCD),
and why, as a friend of mine once found out the hard way, joining 2
null-modem cables trgether doesn't get you an exact straight-through cable.
As regards reading the specs, I've spent too many late nights trying to
convince an HP82164 to do what I want. That's the HPIL-RS232 interface,
and it appears the designer read the specs as regards the correct use of
the flow cotnrol lines, and ingorned the fact that everyone else didn't
follow the standard. The result is a device that follwos the spec and
which is a right pain to actually used.
-tony
If just every
computer manufacturer had read the specs, the world
would have been much nicer,
I'm not so sure. It was inevitable that someone would try to connect
two DTEs together directly, as indeed is easily the commonest today.
It would have been much simpler if everything had been specced the same
and all cables had been what today are loosely called null-modem.