A PLC is a specialized industrial microcomputer. However, when using it to implement the functions of a relay contact control system, it's unnecessary to study it from a computer perspective. Instead, the internal structure of a PLC can be considered equivalent to a relay circuit. A trigger within a PLC is equivalent to a relay, and control is achieved through pre-programmed instructions stored in memory. Therefore, users can disregard the complex internal structure of the microcomputer and memory, viewing the PLC as a controller composed of many relays, whose on/off states are controlled by software; hence, it's called a "soft relay."
Any relay control system consists of an input section, a logic section, and an output section.
The input section consists of control buttons, operating switches, limit switches, phototube signals, etc. It receives various switch information from the controlled object or operating commands from the control panel.
The logic section consists of various relay control circuits designed according to the requirements of the controlled object. The operation of these relays is carried out according to certain logical relationships.
The output section refers to various output devices selected according to user needs, such as solenoid valve coils, various contactors for connecting motors, and indicator lights.
When a PLC is viewed as a controller composed of many "soft relays", its corresponding internal equivalent electrical circuit can be drawn.
The PLC's internal equivalent circuits are connected to the user's input and output devices, respectively. Input devices are equivalent to the signal receiving stage in a relay control circuit, such as operation buttons and control switches; output devices are equivalent to the execution stage in a relay control circuit, such as solenoid valves and contactors.
The equivalent relays provided to users inside the PLC include input relays, output relays, auxiliary relays, time relays, and counting relays.
The input relay is connected to the input terminal of the PLC to receive signals from external input devices. It cannot be controlled by internal program instructions.
The contacts of the output relay are connected to the output terminals of the PLC to control external output devices. Its state is controlled by internal program instructions.
An auxiliary relay is equivalent to an intermediate relay in a relay control system; its contacts cannot directly control external output devices.
A time relay, also known as a timer, has its timing value determined. Once the timer is started, it begins to decrease (or increase) in certain units (e.g., 10ms). When the timer value is reduced to 0 (or increased to the set value), the timer's contacts will activate.
A counting relay is also known as a counter. Once the count value of each counter is determined, and the counter is started, the count value decreases (or increases) by 1 with each pulse until the set count value is reduced to 0 (or increased to the set value), at which point the output contact of the counter actuates.
It is worth noting that the aforementioned "soft relays" are only equivalent relays. There are no actual relays like these in a PLC, and no corresponding current flows through the coil of a "soft relay." Their operation is entirely determined by the programmed instructions.
