> Have a project that the client wants to replace a RS232 with RS485,
pretty simple
> other than these two issues:
>=20
> 1. direction control.  Since the RS485 is 3 wire differential, the
transceivers I
> have seen have direction control pins.  However what happens if you
don't want
> this...ie...several of the converters Ive seen do not see to require
this for a
> point to point setup.  I'm looking at one from FTDI and it uses a Exar
SP481 and the
> two for direction control are floating.

My immediate response is to use CAN drivers. Effectively CAN drivers are
RS422/485 drivers, but instead of actively driving both ways are open
collector drivers, and require a resistor set somewhere on the line to
pull the line the other way.

There is a scheme around for using RS485 drivers in a similar manner, by
driving the enable line instead of the data line. We used this scheme to
get RAD-hard CAN drivers where the only available suitable chips were
RAD-hard RS485 drivers.
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