Protocols — troubleshooting procedures

Reference and troubleshooting guides for industrial protocols: Profinet, EtherNet/IP, EtherCAT, Modbus, OPC UA, MQTT, CC-Link IE, IO-Link, CANopen, and more. Browse by protocol.

12 published procedures.

• MQTT Client Keeps Dropping or Messages Go Missing

  A field procedure for MQTT clients that keep dropping and messages that go missing: how to tell connect failures, keepalive timeouts, duplicate client IDs, QoS downgrades and retained ghosts apart, then how to fix each one.

• Sparkplug B: Stale Data, Missing Tags, and a Client Stuck Out of Sync

  A Sparkplug B consumer showing frozen values or blank tags almost never has a network problem. It has a session problem. Here is how to tell a missed BIRTH from an alias mismatch from a duplicate node ID — and how to fix each without stopping the line.

• PROFIBUS DP Bus Faults: A Field Troubleshooting Procedure

  How to tell PROFIBUS DP faults apart in front of a stopped machine — powered termination, A/B wiring, baud rate versus segment length, stubs, duplicate addresses and GSD mismatches — then fix each one without making the outage longer.

• CC-Link IE: A Station Will Not Join or Exchange Data

  A field procedure for Mitsubishi CC-Link IE networks when a station refuses to join or cyclic data stops moving. Separate wiring faults from configuration faults using module LEDs, then confirm station numbers, the master's network configuration and link refresh in GX Works3.

• CANopen node missing or dropping off the bus

  A field procedure for a CANopen node that never appears, or appears and then drops. Separate bit timing, termination, duplicate node IDs, NMT state and bus-off by what the bus actually does — then fix each one without dropping the machine.

• Modbus RTU on RS-485 Not Responding: Field Troubleshooting Procedure

  A dead Modbus RTU segment tells you one of two things: nothing came back, or something came back broken. Those two symptoms point at opposite halves of the problem. This procedure splits them apart, then fixes each — polarity, common, termination, biasing, frame settings and the one mis-set slave that takes the whole multidrop down.

• EtherCAT Slave Stuck Below OP

  A slave that will not reach OP is telling you exactly what is wrong, if you read the AL status code before you touch anything. This procedure splits the fault into four families, then fixes each — using real registers, counters and ports.

• IO-Link Device Not Communicating: A Field Troubleshooting Procedure

  A dead IO-Link port and a port that was never asked to talk look identical from the outside. This procedure separates SIO mode, validation rejection, data storage conflicts, IODD problems and cable faults in four reads — then fixes each, with the steps that can drop outputs called out before you take them.

• Modbus TCP — Won't Connect, or Connects and Lies

  You will be able to split a Modbus TCP fault into "no socket" versus "wrong map" in under a minute, then fix the Unit ID, the 40001 off-by-one, the wrong function code, or the swapped 32-bit word order — reading the exception code to tell you which.

• EtherNet/IP — Implicit Connection Timeouts and Faults

  Read an EtherNet/IP I/O drop the way the protocol actually fails: a connection that was refused at the door, or one that was accepted and then starved. After this you can name the cause from the fault code alone, size an RPI without overloading a device, and decide whether IGMP is really your problem.

• OPC UA — Client Cannot Connect: The Certificate Trust Handshake

  You will be able to work out, in under ten minutes, which side of an OPC UA connection refused the other and why — untrusted certificate, wrong Application URI, wrong hostname, expired certificate, or a security policy mismatch — and fix it without breaking the clients that still work.

• PROFINET — Device Not Reachable / Not Found

  You will be able to tell, in under five minutes, whether a missing PROFINET device is a cabling fault, a name mismatch, a duplicate name, or a topology problem — and fix each one without touching the IP address.