PROFIBUS DP Watchdog and Min Slave Interval
Two timing parameters quietly protect a PROFIBUS DP bus: the watchdog that decides when a slave should give up on a silent master, and the minimum slave interval that keeps the master from polling a slave faster than it can cope. Set them wrong and you get either unsafe stale outputs or a slave that keeps dropping. This page explains both.
PROFIBUS Watchdog in one line: The PROFIBUS DP watchdog is a timeout each slave runs on its master: if the master stops polling for longer than the watchdog time, the slave assumes the connection is lost and drives its outputs to their configured fail-safe state. The minimum slave interval is the shortest time the master must leave between successive polls of a slave, so it is never addressed faster than it can handle.
The Watchdog Makes a Lost Master Fail Safe
Every DP slave that has the watchdog enabled runs a timer that is reset each time its master polls it. If the master goes silent - a cable fault, a stopped PLC, a crashed master - the timer expires, and the slave concludes it has lost its controller. Rather than holding its last commanded outputs indefinitely, which could leave a valve open or a drive running with no supervision, the slave drives its outputs to their configured fail-safe state. This is the DP equivalent of the safe-state behavior any communications watchdog provides.
The watchdog time must be set with care. Too short and normal variation in the bus cycle, or a brief retry, can trip it and drop a slave that was actually fine. Too long and a genuinely lost master leaves stale outputs commanding the process for too long before the slave reacts. The right value gives comfortable margin over the worst-case healthy cycle time while still reacting quickly enough for the process, and it is a real commissioning decision rather than a default to leave untouched.
The Min Slave Interval Paces the Polling
The minimum slave interval is the other side of the timing coin: it is the shortest gap the master must leave between two consecutive message cycles to the same slave. Some slaves need recovery time between polls to process the last exchange before they can handle the next, and polling them faster than that causes missed responses and errors. The parameter, declared for the device, forces the master to pace itself so it never outruns the slowest slave on the bus.
In practice the minimum slave interval interacts with the overall bus cycle: a slave that demands a long interval slows how quickly the master can revisit it and can stretch the cycle, in the same way a slow slave lengthens the token-and-poll sweep. Getting these parameters from the device's GSD file and honoring them is what keeps a slave stably online. When a device keeps dropping and rejoining, mismatched watchdog or interval settings are a prime suspect, alongside wiring and address problems. The data these polls carry is ultimately what a PLC hands up to a historian or SCADA layer for trending.
Frequently Asked Questions
What happens when the PROFIBUS DP watchdog expires?
The slave concludes it has lost its master - because polling stopped for longer than the watchdog time - and drives its outputs to their configured fail-safe state rather than holding the last commanded values. This prevents a stale command from controlling the process after a cable fault, stopped PLC, or crashed master. The watchdog timer is reset on every valid poll.
What is the minimum slave interval for?
It is the shortest time the master must leave between successive polls of the same slave. Some slaves need recovery time to process one exchange before handling the next; polling faster causes missed responses. The parameter, declared in the device's GSD file, forces the master to pace itself so it never outruns the slowest slave on the bus.
Why does a slave keep dropping and rejoining the bus?
Mismatched timing parameters are a prime suspect. A watchdog set too short can trip on normal cycle variation and drop a healthy slave, while too aggressive a poll rate versus the minimum slave interval causes missed responses. Wiring faults and duplicate addresses cause the same symptom, so confirm the timing settings from the GSD file alongside the physical layer.
Sources and verification
This page references the vendor products and their official documentation published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
- Rockwell Automation Literature Library (Allen-Bradley, Studio 5000) - Rockwell Automation
- Siemens SIMATIC and TIA Portal documentation - Siemens
Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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