PROFIBUS LOOP RESISTANCE TEST 

Loop resistance is the resistance of a cable when you short two conductors on one end, and measure the resistance on the other end between the two wires. Profibus has a loop resistance of 110 ohms/km. When designing high speed Profibus networks (12Mbit/s), the segments must be less than 100m. A segment is part of a network between Profibus Repeaters. A 100m segment will have a loop resistance of 11 ohms. A segment shall not exceed 11 ohms.

To measure the loop resistance you need to turn off the control panel. Go to the upstream repeater of the segment being measured. Short the Red to Green. Go to the repeater on the other end of the network segment. Measure the resistance across the wires.

It is best to disconnect the Profibus wires from the repeater to make the measurement. This ensures that the cable is the only path that is being measured.

When a repeater is being used to branch off, ex, A2B2 and A2’B2’ being used, it is best to measure the two sub-segment separate, and then add the resistance together to get the entire resistance of that segment.