When an MS/TP segment is not working, the classic checks are: 120 ohm terminations at both ends, bias resistors (typically 680 ohm) installed on one device, a unique MAC per device, the same baud rate on every device, and correct cable polarity.

A pragmatic MS/TP troubleshooting checklist runs through six items: (1) 120 ohm terminations present at both physical ends of the segment, (2) bias resistors fitted on a single device (typically the master) so that the idle line is correctly polarised, (3) unique MAC addresses with no duplicates, (4) identical baud rate on every device, (5) correct polarity (e.g. blue/white pairing respected), and (6) a 120 ohm-impedance twisted-pair cable. Around 80 % of MS/TP issues encountered in the field can be traced back to one of these six points.

Before reaching for a protocol analyser, measure the differential voltage A-B at rest with a multimeter: a value far outside the expected 200-700 mV band almost always points to a termination or bias problem.