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The difference between programmable controller and relay control

Programmable Controller and Relay control

In the PLC programming language, the ladder diagram is the most widely used language. The ladder diagram is converted into a PLC-acceptable program by the PLC’s command system, and is entered into the PLC user memory area by the programmer. The ladder diagram is very similar to the relay control schematic. The main reason is that the invention of the PLC ladder diagram is roughly along the component symbol of the user relay control circuit, which is only slightly different.

The main differences between PLC and relay control are as follows:

(1) Different device components

The relay control circuit is made up of many real hardware relays. The PLC is composed of many “soft relays”. These “relays” are actually triggers in the memory and 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; and “soft relays” have an infinite number of contacts that can be programmed because the trigger state can be used any number of times.

(3) Different control methods

The relay control is realized by the hard wiring between the components, so the control function is fixed in the line, so the function is specific and inflexible; and the PLC control is solved by software programming, as long as the program changes, the function It can be changed and the control is flexible. Because the PLC works through the cyclic scan, there is no interlocking and interlocking circuit in the relay control circuit, and the control design is greatly simplified.

(4) Different working methods

In the relay control circuit, when the power is turned on, each relay in the line is in a restricted state, and the combination is closed. In the ladder diagram of the PLC, each “soft relay” is in the cyclic cycle scan on. From the objective point of view, each “soft relay” is subject to the condition, and the on-time is short. That is to say, the working mode of the relay in the control is parallel, and the working mode of the PLC is serial.