Long question, long answer !

 Some time ago, I experiment the receiver problems when I connected
on RS422 ( which is the almost the same as RS 485 ) some control
systems using 80c552 ( an expanded version of 8051) and a PC. The
protocol was like this : the PC queries the stations one at the
time.  The PC transmitter was activated, it sends an address ( that
all the stations listen), then it was deactivated and the PC
listens for the response.

        It appeared the following problem : in the idle periods of
the bus ( when neither transmitter was enabled ) the bus floats
and the receiver TTL output ( which are all enabled) goes in an
impredictable state ( because of the input histeresis) or even
oscillates.
        The solution I found was like this : I prepolarised the
bus so it keeps the polarity when neither transmitter is enabled. I
used two solutions :
1. Two 1k resistors, one from the + line to VCC, other from - line
to GND ( VCC and GND of the RS422/485 driver ). This gives an
aprox. 120 mv between lines when the bus is idle
2. I replaced one 100 ohm  terminator resistor with 2x200 ohm
resistors, one from + line to VCC, other from - line to GND.
This gives an 1V voltage between lines in the idle periods.

                I hope it is useful for you,

                                        Dorin

---------------------------------
Dorin Dogaroiu
Research Institute for Computers
Bucarest, Romania
Email : dogaroiud@pcnet.pcnet.ro
        ddorin@kappa.ro
---------------------------------


  >This is pretty long...  >
>I have two boards with 16c63 parts connected to LTC485 485 drivers.  Pin C5
>is used to enable the driver.  The receiver is enabled at all times.
>
>Any ideas?
>
>Thanks,
>
>Kevin Russell
>kevin.russell@null.net
>
>