EMI effect on modbus communication casuses failure of system



I have a compressor and PC communicating in modbus. The distance between master and slave is about 1000ft. I want to analyze the effect of interference on the communication. In actual field we are facing some issues due to the interference. I want to analyze how much error coming due to the interference . Please suggest any methods. Already I have made bursts from relay chattering and coupled to data line. I could see some flush error in the log file. When I coupled white noise, I could see time out error for some instants. please suggest some more ideas


I assume that at 1000 feet it is an RS-485 network.

Disconnect the shield ground.
Ground the shield at multiple locations along the length of the cable run to induce ground currents.
Put a capacitor between shield and each drive line to induce noise directly onto the signal lines.
Lay a long extension cord directly alongside the cable, directly next to it, to induce noise parallel coupling and run a universal motor tool with brushes, like a big drill or jack hammer. Or better yet, welding cable from an electric arc welder.

Some links to notes on RS-485

PROFIBUS: Grounding Tips, Shielding, Noise, Interference, Reflections, Repeaters and more

AN-979 The Practical Limits of RS-485, Texas Instruments, May 2004

Create a long stub (a long lead line from the multidrop drive lines to the device connections, or a branch in a star topology) that will create a reflection. (TI note has a section on stubs)

IEEE paper: Overcoming data corruption in RS485 communication Related Articles (purchased)
Look at figure 5 to see if breaking the shield offers a way to induce noise.