1. Switch input is the connection channel between PLC and on-site detection components (such as operation control buttons, travel switches, proximity switches, pressure relays, etc.). ）Convert relevant signals reflecting the production process into digital signals that can be received by the CPU unit in the form of switch outputs.

In order to prevent various interferences and high voltage from invading the PLC and affecting its reliability, electrical isolation and anti-interference measures must be taken. In industrial sites, due to various reasons, DC or AC power can be used, and programmable controllers should provide corresponding DC and AC input interfaces.

2. Switching output is the function of converting analog or digital signals detected by sensors into switching signals and then outputting them. The output of the main switch is realized by a relay.

Classification of switch quantities:

1. The switch quantity includes on/off signals and passive signals, and the resistance testing method is resistance 0 or infinity; It can also be an active signal. The scientific name is step signal, which is 0 or 1 and can be understood as pulse. Multiple switch values can form a digital quantity.

2. Digital quantity: A signal type composed of 0 and 1, usually an encoded regular signal. The relationship between analog quantity and analog quantity is the quantified analog quantity.

3. Analog quantity: continuous signal quantities such as voltage and current. Analog signals are signals whose amplitude continuously changes over time, and are digital quantities that have been sampled and quantized.

4. Pulse quantity: The signal quantity of voltage or current that instantaneously jumps from one value to another. After quantization, its continuous change is a digital quantity. If it changes from 0 to a fixed value and remains unchanged, it is a switch quantity.