Source for MMJ cables.

Tony Duell ard at p850ug1.demon.co.uk
Mon Dec 15 16:30:00 CST 2008


[RS232 cables]

> > 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.

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




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