S7-1500 OB86 Rack Error: PROFINET device keeps dropping off, why?
I'm getting an OB86 Rack Error on my S7-1500, and a PROFINET device keeps dropping off the network and then returning. The CPU stays in RUN. What should I check first?
An OB86 Rack Error on a Siemens S7-1500 indicates a failure or return of a DP master system, PROFINET IO system, DP slave, or IO device. Unlike S7-300/400, the S7-1500 CPU defaults to staying in RUN mode if OB86 is not configured, meaning the station's I/O becomes unavailable but the CPU doesn't stop. Repeated failure/return events, as indicated by the OB86 call, often point to intermittent issues rather than a completely dead device.
Likely causes and checks
1Network Interruption
- Points to it
- Intermittent network issues like a faulty cable, connector, or switch port can cause a PROFINET IO device to repeatedly drop off and return to the network. This often manifests with a Fault_ID 16#CB.
- Check
- Inspect the cabling, connectors, and switch ports along the path to the affected device. Additionally, review the PROFINET port diagnostics for any error indications.
2Device Lost Power
- Points to it
- The PROFINET device might be experiencing intermittent power loss, causing it to fail and then return when power is restored. This can coincide with supply events and may be indicated by the device's LEDs turning off.
- Check
- Verify the device's 24 V power supply and its associated protection circuit for any inconsistencies or failures.
3Device Name, IP, or Configuration Mismatch
- Points to it
- If a device was recently replaced or reconfigured, a mismatch in its name, IP address, or module configuration compared to the project settings can lead to communication issues and OB86 errors, potentially with Fault_ID 16#CD or 16#CE.
- Check
- Compare the device's configured name, IP address, and module setup against the current project configuration in the PLC.
Next check: Since the CPU stays in RUN, also verify how the program handles the loss of I/O from the affected station to understand the process impact.
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