Re: Effect of a rs485 damaged driver

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 09/23/04 

Date: Thu, 23 Sep 2004 10:16:02 -0700

Tim Shoppa wrote:

> "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.

Asynchronous serial receivers can detect framing errors, and any
protocol should include at least a checksum on messages -- you can alarm
on either bad checksums, framing errors, or the line going silent. 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Relevant Pages