Modbus Terminators

P

Thread Starter

Paul B

I have a modbus network that is constantly crashing. There is a 120ohm resistor at each end if the network but is there such a device as an Active terminator that we increase the voltages levels of the bus as in Profibus.
 
How many devices do you have hooked up to the network? I am assuming you are talking about modbus serial (as opposed to Modbus TCP ) which is [I believe] RS485.

How are your devices hooked up to the network cable? It is important to keep serial network "stub lenghts" very short. In other words you would ideally chain from one device to another right inside the connector backshell, or a very small trace on the board of your device that shoots to an adjacent connector.

I'm not sure about modbus in particular but it should be limited to a certain number of devices with a certain stub length that you should not exceed.

I would imagine that you can purchase a repeater module if you have to. Check out the fieldbus guys like Hilscher and such. B&B electronics might have something too, their support is great give them a call.

KEJR
 
Your issue may be with the physical network. I suggest that you go to B&B Electronics www.bb-elec.com and get their white paper on RS-485 Troubleshooting. Loading, distance and even the use of a terminator are factors that can "compromise" a network. Mike Fahrion of B&B has written several articles, application notes and technical guides on how to build robust RS485 networks.
 
Can you see if a Tx line is being held ON continuously (oscilloscope?)

I just read about a multidrop network where one slave's driver was staying ON after it should have turned off, preventing replies and subsequent polling.

Bad device was isolated by turning off one device at a time.

 
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