I have many devices on a RS485 bus.
When I disconnect power from one device,
the rest of the (powered-on) devices seem to communicate with each other just fine.
At least it seems to be working today.
Is that a standard feature of most RS485 transceivers — when the power goes off, let the other devices on the bus keep talking?
Or am I lucky today, and there is something else I need to do (what?) to guarantee the powered-on devices can continue to communicate tomorrow, even when powered-off devices are connected to the bus?
Is a RS485 transceiver that specifically spells out in its datasheet it's worst-case load on the bus, when the chip loses all power?
(I don't see any such specification in the datasheet for the
Exar SP485E (datasheet) transceiver
I'm using today,
but perhaps I'm overlooking something?)
(My understanding is that RS485 is specifically designed to run on long cables between boxes that are unlikely to both be turned on and off simultaneously, unlike the bus in How to interface with a bus that's powered when the rest of my circuit is off which was designed for all-or-nothing everything-on or everything-off systems).
Best Answer
Some devices specify high impedance is maintained in power off condition, e.g. http://www.linear.com/product/LTC1485.
Some others aren't so clear, but if you look at the equivalent output circuit for some devices (i.e. http://www.ti.com/lit/ds/symlink/sn65hvd3082e.pdf ) you will find that there is a Schottky diode in series with the output FET. This ensures that when the output pin is driven and higher than VCC or lower than VDD doesn't allow reverse conduction through the output FET so should maintain high impedance whilst the differential is lower than the breakdown voltage of the Schottky diode.
Nick