PLC

A programmable logic controller executes deterministic control logic for a machine or a process skid: reading inputs, running a sequence, driving outputs, and enforcing interlocks on a fixed scan cycle. Its determinism is why it is used where timing matters — a sterilisation hold, a filling sequence, an emergency stop — and why its behaviour is verifiable in a way a general-purpose computer’s is not.

For validation, what matters is the software it is actually running rather than the fact that it is a PLC. A controller executing configured standard blocks is a different proposition from one running bespoke application code, and this estate’s own rule applies: a GAMP category is a property of an implementation, not of a product class. The scoping question is what this controller does for this process, and how much of that is custom.

Its records need particular care, because a PLC often has no native audit trail. Parameter changes, recipe edits and forced I/O may leave no trace unless a supervisory layer captures them — and a forced input left in place after commissioning is a control that has been silently disabled.

• —Deterministic scan-cycle execution is why it is used where timing is part of the control.
• —Validation scope follows what the software does, not the device class.
• —A GAMP category describes an implementation, never a product class.
• —Often has no native audit trail — parameter and recipe changes need a supervisory capture.
• —Forced I/O left after commissioning is a disabled control that nothing reports.

EU GMP Annex 11 (computerised systems); ISPE GAMP 5 (2nd ed., 2022) for software categorisation and the risk-based approach; IEC 61131 for programming languages.