modbus ethernet communications problem

[vsingh]

We are having a lot of trouble with modbus ethernet communications. We are using Lookout 6.0.2. Communications is via a Lantronix UDP100 convertor to RS485. The protocol is MODBUS RTU. A number 6-8) instruments are daisy chained on the RS485 (2-wire) line and then connected to the Lantronix. I am using your Ethernet modbus driver.

If all the modbus instruments are connected then the system works fairly well. However if one gets powered off or disconnected we start getting random comm fails from all the other instruments on that RS485 link. This happens even if I stop polling the dead instrument. It only goes away and the network performs perfectly if I close the modbus driver for the dead instrument.

This behavior does not happen if we use RS485 to Lookout instead of the Lantronix+ethernet.

Need some ideas soon.

- VS

[Ryan.S]

Does the convertor you said support Modbus? Because Modbus serial protocol is different from Modbus TCP protocol, if the convertor doesn't support Modbus, I think it won't work.

On the market, you can find Modbus/TCP to Modbus RTU gateway. But I'm not sure if your convertor has the same feature.

[vsingh]

The converter supports MODBUS. As described, it works fine UNTIL one device is shut off, then everything misbehaves until the dead unit driver file is closed.

- VS