PLC is a special computer, but use it to achieve the function of contact control system, you do not need to study from a computer point of view, but equivalent to the PLC's internal structure as a relay circuit. A trigger inside the PLC is equivalent to a relay, which can be controlled by pre-programmed and stored in memory. Therefore, the user can ignore the complex structure inside the microcomputer and the memory, the PLC is composed of many relays as the controller , but off of the relay is controlled by software, so called "soft relay."

Any one of the relay control system, are by the input part, the logic part and the output part.

The input part is composed of some control buttons, operation switches, limit switches, photocell signals, etc. It receives various switch information from the controlled object or operation command on the console.

Logic part is based on the requirements of the controlled object design and various relay control circuit, the relay's actions are carried out by a certain logic.

Output section refers to the user needs and choose a variety of output devices, such as the solenoid valve coil, connected to the motor of various contacts , lights and so on.

When considering PLC as a controller consisting of many "soft relays", you can draw its corresponding internal equivalent circuit.

PLC's internal equivalent circuit, respectively, with the user input devices and output devices connected. The input device is equivalent to the signal receiving part of the relay control circuit, such as the operation button, the control switch, etc .; the output device is equivalent to the implementation of the relay control circuit, such as solenoid valve, contactor and so on.

In the PLC to provide users with equivalent relay input relay, output relay, auxiliary relay, time relay, counting relays.

The input relay is connected to the input terminal of the PLC and is used to accept signals from external input devices. It can not be controlled by internal program instructions.

The contacts of the output relay are connected to the output terminals of the PLC to control the external output device. Its status is controlled by internal program instructions.

Auxiliary relay is equivalent to the relay control system in the middle of the relay, the contact can not directly control the external output device.

Time relay, also known as the timer 0 timer after the timer value is determined, as soon as the timer starts, in a certain unit (eg 10ms) to start decreasing (or increasing), when the timer is set to the time value is reduced to 0 (or increased to the set value), the timer's contact will operate.

Counting relays, also known as counters. After the count value of each counter is determined, a pulse is started each time the counter is started. The count value is subtracted (or added) 1, until the set count value is reduced to 0 (or increased to the set value), the counter output contact action.

It is noteworthy that the above "soft relays" are only equivalent relays. There is no such actual relay in the PLC. No corresponding current flows in the "soft relays" coils, and their work is completely determined by the programmed program.

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