Ladder diagrams are the most widely used programming language for PLCs. The PLC's instruction set converts the ladder diagram into a program that the PLC can accept, which is then input into the PLC's user memory by a programmer. Ladder diagrams are very similar to relay control schematics, mainly because the invention of PLC ladder diagrams largely followed the component symbols of user relay control circuits, with only minor differences in a few places.
The main differences between PLC and relay control are as follows:
(1) Different components
Relay control circuits consist of many actual hardware relays. PLCs, on the other hand, consist of many "soft relays," which are actually triggers in memory that can be set to "0" or "1".
(2) The number of contacts is different
Hard relays have a limited number of contacts, typically only 4 to 8 pairs; while "soft relays" have an unlimited number of programmable contacts because the trigger state can be used any number of times.
(3) Different control methods
Relay control is achieved through hard-wiring between components, thus its control function is fixed in the circuit, resulting in a specific and inflexible function. PLC control, on the other hand, is achieved through software programming; as the program changes, the function can be changed accordingly, providing highly flexible control. Furthermore, because PLCs operate through cyclic scanning, the interlocking and locking circuits present in relay control circuits are absent, greatly simplifying the control design.
(4) Different working methods
In relay control circuits, when power is on, all relays in the circuit are in a constrained state; those that should close are closed, and those that should close are closed. However, in a PLC ladder diagram, each "soft relay" is in a periodic, cyclical scanning state, meaning that each "soft relay" is subject to conditions and its connection time is brief. In other words, relays operate in parallel, while PLCs operate serially.
