Stationary does not always mean ready
A carriage has stopped. Its velocity reads zero, but a new move is rejected. If MC_Stop still holds the axis, the state can remain Stopping. Standstill has a different command-permission meaning. Read the state before deciding that the controller is ignoring your move.
The PLCopen single-axis state model describes how operations take control of an axis. It is separate from your machine sequence: a machine step called Loading can contain an axis in Standstill, DiscreteMotion or ErrorStop at different times.
| State | Typical meaning | A question before the next action |
|---|---|---|
| Disabled | Axis drive enable is absent | Is power permission available? |
| Standstill | Enabled, with no active travel operation | Is the coordinate valid and the machine ready? |
| Homing | A reference operation is executing | What ends the search, and what bounds it? |
| DiscreteMotion | A positioning operation is executing | Which instance owns the target? |
| ContinuousMotion | A continuous travel operation is executing | Which command will terminate it? |
| SynchronizedMotion | Axis follows a master relationship | How will it engage and leave synchronization? |
| Stopping | Stop operation is decelerating or holding control | Has Execute been released? |
| ErrorStop | Axis reports a fault state | What caused it and what recovery is permitted? |
TryPLC's current single-axis runtime does not implement SynchronizedMotion. The gearing and cam benches are mathematical models, so their drawings must not be interpreted as that state being executed by the ST engine.
Enable is level-driven
MC_Power uses Enable. Call it cyclically with the required enable permission. Its Status reports the runtime's power-enable result; consult the vendor for any separate direction-enable inputs, regulator-enable signals or drive status conditions.
A motion request should depend on readiness, but readiness also includes machine conditions outside MC_Power: a valid coordinate, tooling position, permission to enter the destination and the sequencer's ownership of the axis. MC_Power is not a safety permission and does not verify a clamp.
VAR
FeedAxis : AXIS_REF;
PowerFeed : MC_Power;
MoveFeed : MC_MoveAbsolute;
EnableRequest : BOOL;
MoveRequest : BOOL;
END_VAR
PowerFeed(Axis := FeedAxis, Enable := EnableRequest);
MoveFeed(Axis := FeedAxis,
Execute := MoveRequest AND PowerFeed.Status,
Position := 200.0, Velocity := 100.0,
Acceleration := 200.0, Deceleration := 200.0);This listing shows call order and binding, not a complete machine controller. If a request stays high while Status changes from false to true, the expression passed to Execute can produce a rising edge. Decide whether your machine permits that delayed start. A latched operator request needs an explicit cancellation path.
Refusal and axis fault are different
A move with an invalid parameter can report Error without the physical drive being faulted. A drive fault can place the axis in ErrorStop. Record function-block ErrorID and the separate axis or drive fault information before choosing a reset action.
MC_Reset clears an axis fault through the vendor's supported recovery operation. It does not repair a cable, release a mechanical obstruction or establish a lost home reference. Repeatedly executing Reset every scan can hide the event sequence and cause unwanted repeated attempts.
Bench investigation
Issue Move Execute while the drive is disabled, then advance one scan. The bench reports ErrorID 1, its local code for an unpowered axis. These small teaching error numbers are not Beckhoff, Siemens or CODESYS diagnostic numbers. Enable the drive, lower Execute, advance a scan and raise it again. A new rising edge retries the command.
During motion, activate MC_Stop Execute. After velocity reaches zero, observe Stopping. Lower Stop Execute and advance scans before requesting another move. The bench deliberately demonstrates the lock; it does not model all vendor Busy-release timing.
Recovery worksheet
For a faulted feeder, record the last requested station, current feedback, current state, command outcome and product location. Then define the conditions for clearing the fault and for permitting a separate restart. Keep those actions separate. A cleared fault should not automatically start travel just because an old request remains true.
References
PLCopen specifications contain the state diagram. Beckhoff general motion rules explain function-block lifecycles and implementation details.