Hi,

I am currently working on a network of PIC controlled units for
minitoring machines in a manufacturing plant.  The units will connect to
a 485 bus and will be interogated by a PC (master/slave setup).  I am a
bit unsure about the practical aspects of such a bus in an industrial
environment since this is my first attempt at it.  I would appreciate any
comments from people who have done this before:

I was thinking on routing the bus through each slave unit.  You would
have a female plug going in and a male plug going out.  If the unit needs
to be removed, then the two parts of the bus can simply be plugged into
each other.  The alternative is to use T-style taps from the bus, but
I am not sure how to make the T connections easily and reliably.  Also,
is DB9 plugs/sockets acceptable in the industrial environment?

I have read in an 485 application note, that it is advisable to tie
the grounds of all your nodes in a 485 system together to limit common
mode problems.  Is this usually done?  Can I use a screened two wire
cable and use the screen to tie the grounds?

How critical is the 32-nodes-per-bus limit?  I am planning on using
repeaters, anyway.

What kind of protection is advisable at the input of each slave?  I was
thinking about back to back zeners (say 6.8V) from A and B to ground to
limit the voltage on each line to +-(6.8+0.7)V.  Is a zener good enough
or should one use a Tranzorb or MOV?

I am communicating at 1200Baud.  Is termination really necessary?

Any comments (prefferably from practical experience) would be welcome!

Regards,
Niki