HMI (operator panel)
A touchscreen/operator interface that lets people monitor and control a machine or process.
What it is
An HMI (Human-Machine Interface) is the screen operators use to see what a machine is doing and to control it — start/stop, setpoints, alarms, and status.
How it works
The HMI communicates with the PLC (usually over a network or serial link), reading data to display and sending commands back. It doesn't control the machine directly — it's a window onto the PLC.
A blank or frozen HMI doesn't mean the machine is stopped: the PLC may still be running the process. Treat the screen as a separate device that can fail on its own.
Where it's used
Mounted on panels and machines for operator control and monitoring. Faults are usually power (its supply/fuse), comms loss to the PLC, a software hang needing restart, or a failed display.
Related faults
HMI / touchscreen blank or frozen
The operator HMI/touchscreen is blank, frozen, or unresponsive — operators can't see status or control the machine, even though the PLC may still be running.
PLC communication fault (network / remote I/O)
The PLC has lost communication with a device, remote I/O, HMI, or network — comms-fault indication, missing data, or remote I/O dropping out.
No control voltage in the panel
Nothing in the control circuit will operate — contactors won't pull in, indicators are dead, the PLC may be off. The control voltage that should be there simply isn't.