What causes Beckhoff TwinCAT 3 (XAR/XAE) with EtherCAT warning WcState?

1. A slave not in OP (INIT/PREOP/SAFEOP) or not present: Read the State of every slave in the online view and start with the first one not in OP. 2. Frames corrupted or lost on the line (cable, connector, shielding, EMC): Read CRC counters per port and follow them to the first port that counts; check cable, connectors and shield per ETG.1600. 3. Power loss or reset of a device or segment: Check the supply of the coupler or device and the lost link counters.

EtherCAT Working Counter Error (WcState = 1): Causes

Each slave that processes a datagram increments its working counter; the master knows the expected value. If it comes back wrong, the master cannot trust the data and discards all input data of that datagram: every slave in it shows WcState = 1 (invalid real-time communication) and FrmXWcState sets the bit of that command. Because one datagram serves several slaves, WcState does not say which slave failed; the slave State variable does.

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When it happens

In OP, intermittently or permanently
At startup while slaves are not yet in OP
When a device is switched off, unplugged or removed (also with HotConnect groups)

Likely causes and checks

1A slave not in OP (INIT/PREOP/SAFEOP) or not present

Points to it: One slave’s State shows 0x__1/0x__2/0x__4 or 0x0100 "slave not present"; DevState bits 8–11 set.
Check: Read the State of every slave in the online view and start with the first one not in OP.

2Frames corrupted or lost on the line (cable, connector, shielding, EMC)

Points to it: CRC / frame error counters rising on one port; LostFrames on the master.
Check: Read CRC counters per port and follow them to the first port that counts; check cable, connectors and shield per ETG.1600.

3Power loss or reset of a device or segment

Points to it: Lost link counters incremented; slave State shows link errors (0x0200/0x0400).
Check: Check the supply of the coupler or device and the lost link counters.

Telling it apart

WcState = 1 on many slaves at once is usually one failing slave in the same datagram; Beckhoff recommends separate SyncUnits where working counter errors are expected (e.g. HotConnect).
NC axes see this as error 0x4655 (I/O data invalid for more than n NC cycles); the encoder may lose its referenced state.
Occasional single CRC counts are normal (bit error rate ~10⁻¹²); a fast or burst increase points to a physical problem.

Relevant parameters

Parameter	Name	Why it matters
DevState	Master device state	Should be 0 in operation; bits 8–11 slaves in INIT/PREOP/SAFEOP/error, bit 12 DC not in sync.
FrmXWcState	Frame working counter state	One bit per command in cyclic frame X; should be 0.

Evidence to collect

EtherCAT online tab screenshot (State, WcState, CRC counters per port, LostFrames)
Scope View trace of WcState, DevState and FrmXWcState
Logger export around the event
Topology with port numbers and cable runs

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Beckhoff TwinCAT 3 (XAR/XAE) with EtherCAT warning WcState: Working counter error (WcState = 1, FrmXWcState ≠ 0)