Modbus RTU/RS485

Hi All,

Currently we are facing a issue with serial connection: We have a serial card configured with RS485 half duplex using Modbus RTU to connect to 5 slaves (daisychained). The end has 120ohm termination resistor. The total cable length of the segment is less than 50m. Baudrate is set to 38400. The slaves are WAGO 750-315/300. 

When all devices are connected, only the first device is communicating. The rest are shown are Not Communicating on Diagnostics.

When we unplug the 4th device on the segment, the first 3 devices on the segment communicates well. We can never get the 5th device to communicate. During FAT, all devices were communicating..

The timeout is set at 1second but i tried 3 seconds too. No difference. Also tried increasing retry count to 3 but no improvement.

The Wago fieldbus connectors have resistor switches on it but does not make a difference ON or OFF.

We replace the Wago fieldbus connector on the 4th device but did not improve.

Any clue whats happening?


Thank you.

1 Reply