Non-Isolated and Isolated Modbus Slaves on Bus


Thread Starter

Beavis Cleavis

On a Modbus network, I have (1) multiple devices that provide isolation from power but not isolation at the 485 output and (2) multiple devices that are isolated from the power and the 485 output. This group of devices (1 and 2) is monitored by a Red Lion touch interface.

The (1) devices provide a 10 ohm resistor between the RS485 ground and isolated ground from the power. These devices are daisy chained together. The last device of (1) is then daisy chained to the first device from (2).

Does anyone see any problems with putting an isolator/repeater at the beginning and end of (1) devices? That is, one repeater/isolator between the Red Lion and the first device in (1) and one repeater/isolator between the last device of (1) and the first device of (2)?

Without a third wire in my cable, what are some options to ground the RS485 isolated ground to at least get things up and running?

Any ideas/suggestions are appreciated.