Roger Weichert wrote:
> I'm working on some gear made by DeLaval which uses what they call the
> "Alcom Bus" to communicate back to a central control.
>
> It is a simple 2-wire system that daisy chains from one unit to the
> next, and requires a 150 ohm termination resistor at each end. There
> is also a comment in the operation manual about NOT using shielded
> cable.

I've never heard of "Alcom bus".  My guess from your description is that
it's RS-422 with their own protocol on top of that.  That fits with the
chain topology, the need for terminators at each end, and the 0-5V signal
range.  RS-422 is basically a differential pair with each wire 0-5 volts.
There are off the shelf line driver and receiver chips for that, so that's
most likely what they used.

The requirement to use unshielded cable is probably to guarantee symmetry
between the two lines.  If you use two conductor shielded cable, like common
audio cable, then the outer conductor picks up all the external noise, which
then becomes a differential signal.  In a twisted pair, both wires pick up
the same external noise so that it becomes a common mode signal, which the
line receivers are specifically designed to ignore within a certain "common
mode range".  Cable with a twisted pair inside and a third conductor as the
shield would be fine for preserving the symmetry between the two data lines,
but would introduce ground loops that they may feel installation technicians
wouldn't know how to deal with.  It's also more expensive cable, so maybe
they weren't thinking of that anyway.

As for the protocol, most likely that is proprietary.


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