PROFIBUS troubleshooting

Start in the master

The PLC diagnostic buffer and station status show which slaves fail and how: station failure (no answer), configuration fault (GSD/modules do not match), parameter fault, or device-specific diagnostics (channel errors). The standard slave diagnosis contains station status bytes, the master address and the ident number; extended diagnostics are described in the GSD.

Pattern → probable cause

Measurements with power off

On the disconnected cable, between data lines A and B:

• ≈ 110 Ω with both terminators ON (two 220 Ω in parallel);
• ≈ 220 Ω with one terminator ON — the other end is missing;
• open circuit — no termination or broken line.

Also check A and B for short circuits to each other and to the shield, and swapped A/B wiring at a connector. Professional bus testers and oscilloscopes show signal shape and reflections when the simple checks pass.

Prevention

• Document addresses, baud rate and terminator positions on the cabinet drawing.
• Use connectors with diagnostic LEDs or test points where available.
• Route bus cables separately from power cables with the distances given in the installation guideline; bond shields to ground on both ends.