Re: Effect of a rs485 damaged driver

From: Tim Shoppa (shoppa_at_trailing-edge.com)
Date: 09/23/04 

Date: 23 Sep 2004 05:26:03 -0700

"Reginald Jean Louis" <louis_reginaldjean@hotmail.com> wrote in message news:<9Os4d.21940$pA.1489170@news20.bellglobal.com>...
> I have a bunch of rs485 transceivers arrange in a 4-wires bus topology (my
> question is good for 2-wire/half duplex too). I want to know if a blown up
> driver can affect the line by putting permanently the line in a high or low
> state? If so, there is a way to prevent that?

Receivers can latch up/blow up too, for example by shorting one or both
of their inputs to Vcc or ground.

The most common failure I've seen is shorted output drivers holding one
line high or low. Sometimes the system still continues to sort-of work
but not reliably at all.

I've seen some applications guard against this by putting 50-ohm-or-greater
resistors in series between each node and the bus wires. Noise immunity
is decreased but the idea is that a "good" driver will outvote a "bad" driver
that has only one of its outputs shorted to ground/Vcc. This doesn't really
help if the "bad" driver is just ignoring its tristate input and jabbering
all the time as it then (electrically) has just as much vote as a good
driver.

Some bus networks have receivers that can issue an alarm if one of the
bus lines is stuck high/low or they see meaningless jabber. Haven't
seen this for RS-485 although some software will issue somewhat meaningful
alarms when they cannot see their own transmissions :-).

Tim.

Relevant Pages
Quantcast