[ILUG] house wiring - serial over cat-5

[Geoff Moore]

>
>
> Aren't RS422/RS485 the Current loop standards which cope with
> noise much better.
>


Mmm...kinda...

They're differential (two wires per signal, when the voltage on one goes
more positive the voltage on the other goes an equal amount more negative).

That's what makes them cope with noise better. The receiver is designed to
look for differential signals so a common mode signal (ie a signal which
goes positive or negative on both wires simultaneously) or a single ended
signal (ie one that goes positive or negative on either one of the wires but
not the other) will be ignored. Picked up noise will generally be one or the
other. If you put the differential signals onto a twisted pait then it's
likely that any noise picked up in the cable will be purely common mode and
is therefore easy to ignore.

It's not really current loop, that's different again. 20mA is still used to
an extent in more industrial environments and 4-20mA current loop is used
for transmitting analogue signals.

By the book RS422 is differential point to point and RS485 is differential
multi-drop but you can use 422 for multi-drop and 485 for point to point if
you like.



Geoff