PLC (Programmable Logic Controller)

A PLC is a rugged industrial computer built to control machines and processes. It reads signals from the field, runs a stored program, and switches outputs to make things happen — reliably, in real time.

It runs a continuous scan: read all inputs, execute the program logic, then update all outputs — over and over, many times a second. Inputs come from sensors, switches, and signals; outputs drive contactors, valves, lamps, and drives.

An output's indicator LED follows the program's intention, not the physical output — which is why an output can show 'on' while the field device stays dead (a failed output, lost field supply, or broken wire).

Machine control, process plants, building services — anywhere sequenced or interlocked control is needed. I/O can be local or remote, often over a network.

Common faults: inputs not reading despite a signal present, outputs on but device dead, module faults, comms loss, and the controller dropping into a fault/stop state.

The PLC itself rarely dies; its edges do. Input side: a signal present at the terminal but the program never sees it (dead channel, wrong common, threshold issues). Output side: the program says on but the field device is dead — blown output transistor or relay, lost field supply, or a broken wire beyond the terminal.

Above that: module faults and rack errors, comms loss to remote I/O or the HMI, and the controller dropping to STOP/fault — which kills every output at once, not just one.

Split the world at the terminal strip. Inputs: measure the signal at the input terminal itself — present there but no input LED / program bit means the PLC side; absent means the field side. Outputs: command the output on and measure at the output terminal — volts there means the fault is out in the field; nothing there means the card or its field supply.

Check the separate field supply and its fuse early — the card's own LEDs can run happily off the backplane while the field 24 V is gone, which looks exactly like 'everything on, nothing works'.

Output LEDs on many cards show the commanded logic state, not the real output — a blown channel can sit there glowing 'on' over a dead device. Trust the meter at the terminal, not the LED.

Forcing I/O bypasses the program's interlocks — machinery can move the instant you force. Clear it with whoever owns the machine, and remember a 'dead input' can also be a 2-wire sensor leaking just under the input's threshold.