What is the difference between programmable logic controllers and traditional relay logic controllers?

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

(1) Different components

Relay control line is composed of many real hardware relays. PLC is composed of many "soft relays", these "relays" are actually triggers in the memory, can be set to "0" or set to "1". The first one is the "1".

(2) the number of contacts is different

Hard relays have a limited number of contacts, generally only 4 to 8 pairs; and "soft relays" can be programmed for an unlimited number of pairs of contacts, because the trigger state can be accessed any time.

(3) different control methods

Relay control is realized through the hard wiring between the components, so its control function is fixed in the line, so the function of a dedicated, inflexible; and PLC control is solved through software programming, as long as the program changes, the function can be followed by a change in the control of a very flexible. And because the PLC is working through the cycle scanning, there is no relay control lines in the interlock and interlock circuit, the control design is greatly simplified.

(4) work differently

In the relay control line, when the power supply is turned on, the line of each relay are in a constrained state, the close of the close, the break of the break. In the ladder diagram of the PLC, the "soft relay" are in a cyclic scanning cycle connected, from an objective point of view, each "soft relay" by the conditions, the connection time is short. That is to say, the relays in the control mode of operation is parallel, while the PLC mode of operation is serial.