The PLC 3 Series of programmable logic controllers was among the first series of PLCs to be produced by Allen-Bradley in the 1980s. Many machines, factories, and plants still use these devices and there is still a demand for these PLCs and their spare parts because they are reliable and rugged. When they were launched, the PLC 3 series controllers were innovative in industrial automation functions that are applicable to standalone machines as well as to integrated production lines in a factory. The PLC 3 devices brought extraordinary flexibility and modularity to the programmable controller market, and they are designed to support larger automated systems with high-end processors.

The Allen-Bradley PLC 3 systems themselves can have 8 components that they use to acquire data, to send control signals, to deliver power to the connected modules, sensors, and actuators, and to communicate. These 8 main components are the main chassis processor, main processor modules, power supply, front panel, memory modules, industrial terminal, I/O scanner modules, and the expansion system. The main chassis on the PLC 3 system is designed to mechanically protect and group up to 6 modules, and they also have a front panel to check the whole system performance and diagnostics status. The front panels have keyboards and 16-digit alphanumeric displays. The PLC 3 power supplies are responsible for generating + 5 Volts, +15 Volts, and -15 Volts voltages to power all the modules that are installed in the main chassis. The program is loaded into the PLC 3 controllers by using an industrial terminal known as the Allen-Bradley 1770-T4 terminal. The I/O capacity of the PLC 3 systems is 4098 inputs and 4098 outputs.