The PLC 5 control systems are revolutionary inventions that have the ability to be programmed from a personal computer. They were developed to be the most advanced possible kind of industrial devices that are able to replace a huge number of relays while taking up considerably less space than the relays and they need very little supporting material such as wires and accessories. Prior to the PLC 5 devices, PLCs required dedicated computer support, but the PLC 5 series was initially programmed from the DOS operating system on any computer and later when it was further developed through the RSLogix5 software. The Allen-Bradley PLC-5 systems are basically made up of processors that decide how the system will react in certain input states based on the program, a rack that holds modules, I/O modules with different numbers of inputs and outputs, a power supply, and field wiring.

The Allen-Bradley PLC 5 series processors include the most popular models such as the PLC-5/10, PLC-5/12, PLC-5/15, and the PLC-5/25 processors and special variants like the Enhanced, Ethernet, and ControlNet processors. To maintain the modularity of the system and to be ready for any future upgrades, all the processors are the same size, they can all process data from any 1771 I/O modules with up to 32 points per module, support the same programming software and instruction sets, and use ladder and SFCs in programming. This flexible programmable logic controller series allows for the support of classic digital and analog inputs and outputs, communication modules, and special I/O modules for exchanging information with flowmeters, encoders, weight scales, bar-code readers, and RF type devices.