Engineering · Electrical & PLC
Relay Logic vs PLC Control
The same machine logic, wired in copper or written in a program: what changes?
Relay logic achieves control by wiring relays, timers and contactors together. A PLC does the same job in software. The logic is identical, so this page lets you run both versions of the same circuits: the start/stop seal-in, an on-delay timer and a two-motor sequence. It then shows the real differences: how a PLC scans, what a modification costs, how a fault is found, and which one suits which machine.
Press START: the coil energises and its own contact closes in parallel with the START button, so the coil stays on after the button is released. That is the seal-in, or latching, circuit. STOP is a normally closed button wired in series, so it breaks the circuit and also removes the power if its wire breaks. On a PLC the stop button is still wired normally closed, so its input is ON when the button is not pressed, and the program examines it as closed.
Motor 1 starts at once, a timer counts the delay, and Motor 2 starts when it expires. STOP drops motor 1, which resets the timer and drops motor 2 as well. Choose how the circuit is labelled: as relay wiring or as PLC addresses; the logic is the same.
Every relay in a circuit works at the same time, so the order of the wiring does not matter. A PLC reads all inputs, solves the program from top to bottom, then writes all outputs, and repeats. An input that comes and goes between two reads can be missed, and a change can take up to two scans to show on an output. For very fast events use a fast input, an interrupt or a high-speed counter.
| Change | Relay logic | PLC |
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| Point | Relay logic | PLC |
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| Relay circuit | PLC program |
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Emergency stops and other safety functions are not moved into standard PLC logic. They stay hardwired, or use safety relays or a safety PLC, so that a PLC failure cannot prevent the safety function.
| Feature | Relay logic | PLC control |
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| Relay logic | PLC control |
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Scan times, relay response and effort ratings differ between products. The figures here are typical, to be confirmed in the datasheets. Isolate and lock out before working on a control panel.