Symbols of commonly used digital input/output components in PLCs

PLC input/output signals can be digital, analog, or switch signals, with switch signals being the most common. PLC switch input devices come in two forms: passive switches, such as various buttons, relay contacts, and control switches; and active switches, such as various proximity switches, sensors, encoders, and photoelectric switches. PLC switch outputs typically use three forms: relay outputs, transistor outputs, and bidirectional thyristor outputs. Because the load capacity of each PLC output point is relatively small, it can generally directly drive very low-power actuators, or drive contactor coils or indicator lights that control actuators. The symbols for commonly used PLC switch input/output components are shown in Table 1.