(me too)! Re: [Soekris] rs485 instead of rs232

[Henry Spencer]

On Thu, 1 Jul 2004, Poul-Henning Kamp wrote:
> RS-485 is RS-422 in multidrop configuration...

In fact, a lot of RS-422 hardware permits multidrop operation.  It was a
bit tricky, because RS-422 is *not* specified to tolerate simultaneous
enabling of more than one transmitter, accidental or otherwise. 
Simultaneous-enable tolerance was the key capability added by RS-485. 

> Being multidrop, you need to tell the hardware when to drive the
> line and when to go tri-state.  This happens in all sorts of weird
> ways.  Some cards figure out for themselves, some even disable the
> receiver so you don't hear your own transmissions.  Some cards you
> have to wiggle RTS to enable the transmitter, some you have to wiggle
> other bits.

And there are some which drive the line for SPACE but rely on the line
termination for MARK, in which case enabling control is unnecessary.
(A project I worked on used 232-to-485 adapters which did that, which was
very convenient.)

                                                          Henry Spencer
                                                       henry at spsystems.net