Programmable logic controllers originated in the late 1960s in the automotive industry and were designed to replace complicated relay logic systems that were difficult to design, improve and troubleshoot with a single, easy-to-use controller. GM Hydramatic was having problems with reliability and documentation for machines. Their desired solution was a solid-state controller. Richard E. Morley and his company Bedford and Associates presented a proposal with a design for a modular unit with no interrupts based on ladder logic, which uses ladder diagrams that resemble a traditional relay logic circuit.
They called their controller the 084 after their 84th project and established the Modicon company, deriving its name from Modular Digital Control. Modicon is now owned by Schneider Electric.
The PLC provided several advantages over earlier automation systems. It tolerated the industrial environment better was compact and required less maintenance. It was easily extensible with additional I/O modules. With simple programming language focused on logic and switching operations, it was more user-friendly.
In a parallel development Odo Josef Struger helped invent the Allen-Bradley PLC and is credited with inventing the PLC acronym. As PLCs became more widely adopted and multiple competitive systems developed, the need for a unified programming language led to the introduction of the IEC61131-3 standard for PLC software.
As the PLC evolved the focus shifted to simplifying programming and development of human machine interfaces. In the1990s, the widespread implementation of HMIs opened the door for connecting the internet to the factory floor.