PROFIBUS DP Cable Termination and Bus Biasing
A PROFIBUS DP segment that drops slaves, corrupts data, or works only when one panel is powered is very often a termination problem, not a device fault. DP termination is active - it powers bias resistors that hold the idle bus at a defined level - so where you place it and whether it stays energized matters. This page explains DP termination and biasing and why they behave differently from a plain resistor.
PROFIBUS DP Termination in one line: PROFIBUS DP termination is an active network built into the standard bus connector: at each end of a segment it places a matching resistor to stop reflections and, crucially, pull-up and pull-down bias resistors that hold the idle RS-485 pair at a defined logic level. Because those bias resistors need power, the terminator must be energized, so a powered-off end device or switched-off terminator makes the whole bus unreliable.
Termination Does Two Jobs, Not One
On the RS-485 physical layer that DP uses, an unterminated line reflects signals off its open ends, and those reflections corrupt the data - the same reflection problem any transmission line has. Termination places a matching resistor across the pair at each physical end of the segment to absorb the signal and kill the reflection. That is the job most people picture. But PROFIBUS termination does a second job that a bare resistor does not: it biases the bus. This active behaviour is one reason a DP segment is more particular than a casual read of RS-485 serial signaling suggests.
Biasing means holding the idle line at a known logic level when no station is driving it. Between messages the RS-485 drivers let go of the pair, and without bias the line floats to an undefined level where noise can be misread as data. The DP terminator adds a pull-up on one wire and a pull-down on the other, forcing the idle pair to a clean, defined idle state. Termination that only matched impedance without biasing would leave the bus vulnerable in exactly the quiet gaps between the polls of the DP master-slave exchange.
Why the Terminator Must Stay Powered
The bias resistors need a supply to pull against, and that supply comes from the station whose connector is providing the termination - which is why PROFIBUS bus connectors carry a termination switch and why the standard 9-pin D-sub connector routes supply pins through to feed it. Set the switch on at each of the two physical ends of the segment and off on every connector in between. Get this wrong and the classic faults appear: no terminator, or a mid-bus connector switched on, and you get reflections and intermittent errors; a terminator switched on but its device powered down, and the bias collapses so the idle bus floats.
That last case is the one that catches people. A perfectly wired segment can misbehave simply because the device at one end is switched off for maintenance, removing the power its terminator needed to bias the bus. The cure is to terminate at points that stay energized, or to use a standalone powered terminator at a segment end, so biasing survives a single device going dark. When a DP bus shows intermittent slave dropouts that track with which panels are powered, suspect termination and bias before you suspect the slaves - a different failure signature from the turnaround problems seen with RS-485 converters. The specific resistor values and any stub-length limits belong to the PROFIBUS wiring guidelines and the connector manufacturer's datasheet.
Frequently Asked Questions
Why does a PROFIBUS terminator need to be powered?
Because DP termination is active: besides the matching resistor that stops reflections, it includes pull-up and pull-down bias resistors that hold the idle bus at a defined level, and those need a supply. The power comes from the station providing the termination. If that device is switched off, the bias collapses and the idle bus floats, causing intermittent errors even though the wiring is correct.
Where do I switch termination on in a PROFIBUS DP segment?
At the two physical ends of the segment, and nowhere in between. PROFIBUS bus connectors have a termination switch; set it on at the first and last connector of the segment and off on every connector in the middle. A terminator switched on mid-bus, or missing at an end, causes reflections and data corruption.
What is the difference between termination and biasing?
Termination is the matching resistor that absorbs the signal at a segment end to prevent reflections. Biasing is the pull-up and pull-down that hold the idle RS-485 pair at a defined logic level between messages so floating noise is not misread as data. PROFIBUS termination does both, which is why it is active and needs power, unlike a bare terminating resistor.
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