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compressor panel is doted by a serial link between the PLC Allen Bradley Flex IO Prosoft MVI94-MCM module and the VFD panel, the UCP and VFD have separate power supply and the UCP panel has a 24VDC float.

I'm experiencing a problem of floating on the power supply 24VDC on all the UCP by -6VDC every one second, when I disconnect the modbus link to VFD the 24VDC becomes stable and when we put pack the modbus the power starts floating again from 24VDC to 18VDC.

Can you please advise us what we can do to solve this problem.

Here you are the master port configuration:

[Modbus Master]
# Settings required for Master Driver
Command Count: 2 #Command list count for port
Minimum Command Delay: 1000 #mSec delay between commands
Command Error Pointer: 90 #Internal database location of cmd err list data
Response Timeout:1000 #mSec for response timeout of command
Retry Count: 3 #retry count for failed requests
Error Delay Count: 10 #0-65535 delay for slave after comm error

This doesn't sound like a Modbus communication configuration issue. But it isn't clear what problem the cyclic voltage dip causes.

I suspect a grounding problem through what is probably 2 wire RS-485 circuit.

I'd put an RS-485 isolator module in the RS-485 circuit to see if the cyclic voltage dip doesn't disappear.

Yes it is 2 wire modbus communication, the ground is connected, and the ground of the 2 systems is separated,.

yes you are right the isolation may solve this problem, Can you please give more details how to do that.

I appreciate your help.

A two way isolator isolates the lines from each other.
A 3 way isolator isolates the lines and the input power.

I use a B&B Electronics 485OPDRI, but there are other makes and models
http://www.bb-elec.com/product11.asp?sku=485OPDRI
Be sure to take out any space in the URL above (forum sometimes inserts a space or two)