ABSTRACT

Programmable logic controllers (PLCs) initially appeared in the industry during the 1960s and had a completely different form than those implemented in the twenty first century, since they were built out of logical components that only replaced the operation of the auxiliary relays. A PLC contains several dozens of all the necessary classical industrial automation components due to its digital form. A PLC contains a microprocessor that executes all the internal functionalities of the needed automation. PLC manufacturing companies have adopted two basic types of PLC constructions, which are the compact form and the modular shape. For most PLCs, input and output (I/O) modules are one of the most common and simple parts of equipment, but also the most important after the microprocessor unit. Digital inputs modules are designed to receive discrete signals from the input devices and convert them appropriately for further processing, while electrically isolating them before being transferred to the memory of the PLC.