Siemens input-output Card Cpu-DC-plc Input Module with 8 inputs, 24 V DC, galvanically isolated, source/ sink switching for CPU 221/222/224/224 XP/226.

A programmable logic controller (PLC) is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or lighting fixtures. PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result. Figure 1 shows a graphical depiction of typical PLCs.