PROFIBUS RS-485 active terminations (a 390 / 220 / 390 ohm resistor network biased from 5 V) must be present at both ends of the segment and powered; the bus will not work reliably if the terminations are passive or unpowered.

The active terminator is a biased resistor network: 390 ohm between +5 V and DataA, 220 ohm between DataA and DataB, and 390 ohm between DataB and 0 V. The pull-up/pull-down branches force a defined idle level when no device is driving the line, which is essential for the receivers to discriminate the first bit of every frame. The 220 ohm element in the middle handles the line termination itself. The network is usually integrated in the DSUB-9 connector with an ON/OFF switch, and checking that the terminations are correctly powered is the first step of any field diagnostic.

If a fresh segment shows random retries from device 1, unplug the laptop and confirm the two end DSUB-9 connectors have their termination switch ON and the host is powered; an unpowered terminator looks fine visually but biases nothing.