PLC
PLC Fundamentals
Scan cycle, I/O image, and program organization of programmable logic controllers.
Overview
A programmable logic controller is the deterministic execution core of a machine or process cell: it reads field inputs, solves user logic, and drives outputs on a fixed, repeatable cycle measured in milliseconds.
Engineering purpose
Guarantees bounded reaction time and predictable behavior for discrete and hybrid control, where a PC operating system cannot.
How it works
Each scan copies physical inputs into the process image, executes the program against that frozen image, then writes the output image back to the terminals. Interrupt blocks preempt the cycle for time-critical events; the watchdog supervises total scan time.
- A PLC executes a cyclic scan: read inputs, execute logic, write outputs — scan time sets the response latency.
- I/O process images decouple physical terminals from logic; forcing bypasses logic and must be strictly controlled.
- Organize programs into cyclic, startup, and interrupt blocks; keep safety logic separate from process logic.
Common faults
Scan-time overrun from unbounded loops or heavy communication; forced I/O left active after maintenance; logic acting on stale data when modules fail silently; battery/memory loss wiping retentive data on power cycle.
Diagnostic checks
- 1Check the CPU diagnostic buffer for faults and scan-time overruns.
- 2Verify I/O addressing against the hardware configuration.
- 3Confirm retentive memory and clock backup health before any planned power-down.
Safety notes
Never treat standard PLC logic as a safety function — emergency stops and guard circuits belong in certified safety relays or F-CPUs, hardwired per the machine's risk assessment.
Commissioning notes
Force each I/O point once against the loop drawing before logic test; record scan-time baseline at full load and alarm at 80% of watchdog.
Related concepts
Process image, scan cycle, organization blocks, retentive memory, watchdog, I/O addressing.
plc · scan · cycle · cpu · logic controller · پیالسی · اسکن · سیکل · کنترلر منطقی
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When Hermes Brain uses this article
Hermes Brain cites this library when a question involves scan behavior, CPU diagnostics, I/O addressing, or general controller fundamentals across any vendor.
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