Facing an issue with Modbus TCP/IP Communication


Hi Forum,

I am working at a Gas Processing Plant which is running Emerson Delta V Distributed Control System (DCS). The Control System is communicating with over 24 3rd party Packages in the field over Modbus TCP/IP using its VIM Module. We have implemented redundant communication with most of the packages so we have a primary switch and a secondary switch. We are facing a communication issue regarding an Over Head Compressor (using Rockwell Allen Bradley PLC) and Instrument Air Compressor by CAPS Australia running XE Controller (Dedicated Controller). The issue is that when the Over Head Compressor is plugged into the VIM Network (Network in which all 24 3rd Party Packages are plugged), the Instrument Air Compressor stops communicating. We can’t even ping the compressor. At times the Instrument Air Compressor does reply but with TTL of over 30ms. When OVHD Compressor is plugged out, Instrument Air Compressor resumes communication.
I have attached wire shark capture of the network with and without over head compressor. When the OVHD Compressor is plugged in, the capture shows heavy traffic being generated by its PLC (IP address Another physical observation is that before the OVHD Compressor is plugged in the VIM Network, the NTRON Switch (Ethernet Switch) in OVHD Compressor panel shows static lights, unlike lights of a normal healthy network. When the OVHD Compressor is plugged into the VIM Network, all the lights of switches in VIM Network which were blinking previously also become static. Do the static lights mean that the network is being choked due to heavy traffic?
Below are links for the Capture, IP Scheme and Architecture of Over Head Compressor.

Please also note that OVHD Compressor is using following IPs PLC (Internal PLC IP) PLC (Internal PLC IP) HMI (Internal PLC IP) RedLion device for DCS Communication

I would really appreciate if anyone would help me trouble shoot this issue.
If you want more details regarding the architecture, feel free to ask.