Here are some comments based on some years of experience with all types of
RS422 and RS485 equipment.

If you relay through each unit, if one unit fails, the entire bus is
disconnected past that point.  The idea of RS485 is that each unit can talk
on a party line, it tri-states when it is not talking.  If the line is ever
going to be tristated, then it is useful to pull some inputs to the mark
state (i.e. 1K OHm to +5 and 1K Ohm to Gnd on each wire at one or more
receivers).  We often go hundereds of feet with flat cable, just pressing
an insulation displacement DB9 or DB15 on the wire near each connection point.

If you do not terminate the bus, you will get reflections.
    With reflections, you will start to get errors when the speed of light
turnaround time on the bus equals the time of 1/2 bit cell time for
asynchronous or synchronous systems.  For instance this happens at 64,000
bits/second this happens at about 2000 feet.  If the baud rate is slow,
and/or the connection is short, then reliable operation occurs without
termination. So with 1200 baud, you probably will not get errors.  Without
termination, the line will be unbalanced, and considerable RF radiation
will occur (read as: Both signal and reflections radiated are the same as
attenuating the signal, nearby FM radio reception wiped out, if there are
complaints the FCC will be on your case).  It is recommended anyway that
each RS485 pair be in its own shield for the most reliable operation, esp.
near arc welders, etc.

Yes it is advisable to tie the grounds of all units together.  This is
preferably done via a low resistance/impedance separate grounding strap
between all the units.  If you use the ground in the signal cable, the
common mode currents in the cable may lead to signal errors, may be high
enough to heat the small ground wire and melt the insulation.  I have been
shocked while touching ungrounded equipment with one hand and the
cable-connector in the other.   When we use shielded cable, we ground it at
only the transmitter end (Faraday shield).  This is the practice at almost
every telephone company installation in the U.S., one end of the shield is
grounded, the equipment is interconnected with a separate ground strap.

The 32 nodes per bus is based on the current capability of the driver, and
the load resistance it has to drive.  The load is made up of the input
resistances of all the receivers and the far end termination resistor (term
usually = 121 Ohms 1%), all in parallel.  If you exceed 32 nodes, your
driver runs too hot and fails early, and you noise immunity is shot (i.e.
less and less difference between a one and a zero).

When the drivers are tri-stated, they are fairly robust, takes about 17
volts to blow them out.  The zeners to ground are a good idea, we use 9
volt zeners in mountain top relay station applications where lighting
strikes often, and it 'saves' the RS422/RS485 chips almost every time.
With high baud rates, the capacitance of the zeners can become a problem,
so don't get carried away (hi-power=hi-capacitance).  Also, keep the zeners
right on or near the connector (25 mil traces), so if you take a big hit,
the board traces are not damaged or even evaporated.  You don't need back
to back zeners, just regular zeners, whose diode action keeps the signal
from going over 0.7 volts below ground.  If you let the signal go 5 volts
below ground at the receiver with high currents, you can kiss some (not
all) chips goodby.

Regards,  Ron Fial
----------------------------------------

At 10:29 AM 1/22/98 GMT+0200, you wrote:
>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
>

Regards,
  Ron Fial