RS485 current flows into unpower node

Hello experts,
Our RS485 bus connects 10 nodes together. But one of the nodes might be shut down(power off) from time to time. I am wondering if the current flowing on RS485 bus might revert the current back into the powered off transceiver and do damage to the transceiver. Any explicit specification that prevents this from happening?

BRs,
bittware

I have dealt with similar problems with CAN transceivers (which tend to be very similar to RS-485 transceivers). Though there didn't seem to be any protection diodes going to the rails, there was a definite load presented by the transceiver (I believe about 10K in my case).

My solution was to put some low current MOSFETs between the bus and the transceiver. This worked very well.

I should mention that my worry was not that the devices would be damaged, but that they'd load the bus (my application was a very strange one and it needed the bus to be completely unloaded and unterminated at times).

As for doing damage - I can again tell you that some CAN transceivers will be destroyed if the GND pin is floating but there is a potential across other pins. If it is shutdown via a shutdown pin then I'm sure it'll survive.

Can you elaborate more on your application? Also, what transceiver are you using?

All RS485 drivers I know are specified to allow the output to swing above and below the rails, which means, that connecting an unpowered RS485 transceiver won't do any harm and allows the bus to operate regularly. The classical SN75176 e.g. is specfied with -10/+15V at the bus terminals.

A special problem arises, if only the terminating nodes implement bias resistors to set the idle state, as it is e.g. the case with PROFIBUS networks. If the node(s) providing the bias voltage are unsupplied, the bus may fail, because some peers see a break state.


P.S.: CAN drivers have basically a similar voltage range specification, but the bus may be more sensitive to irregular behaving devices due to the recessive "1" state.

RS-422 and -485 are supposed to be high impedance at
power-off. RS-422, only to VDD but RS-485, to both rails.

But that only pertains to the IC, and somebody could still
mess it up at the board / box level by adding clamping that
doesn't have the same properties, or stuff like that.

somebody could still mess it up at the board / box level by adding clamping

Click to expand...

Good point. Suggested surge protection clamping for RS485 would be a TVS to ground. It gives no problems if the node is unpowered.