This is basically where it all started.
The General Motors Hydra-matic division wanted to improve their ability to change their transmission assembly-line system without having to rewire everything. They produced a specification and went to industry for a solution. They had several parties interested and installed prototypes from three.

Standard Machine Controller specification.

The unit will be of modular construction and operate in a factory environment subject to nearby high voltages and vibrations. Built-in isolation would accept 120 vac digital signals and would provide at least sixteen 120 vac, 4 amp outputs. Using solid state components, it would have 32 inputs expandable to 256 and 16 outputs expandable to 128. Stored information or programs would not be altered or lost due to a system power failure of up to 12 hours. It would be easily programmed and re-programmed. It will have at least 1K of memory expandable to 4K. It would be capable of handling eight simultaneous timing functions with a timing range adjustable from 0.1 to 10 sec.

A final form of the solicited “Standard Machine Controller” was completed in early June and was given to four vendors, Allen-Bradley, DEC, 3-I (Information Instruments, Inc), and Century Detroit. Later on, Cutler-Hammer, Cincinnati Milling Machine, and Bedford Associates obtained copies.

Out of this group of seven, only three, DEC, 3-I, and Bedford Associates, delivered prototype controllers for evaluation.

The Results.

Specification reviews, board and hardware designs, proposal preparations, building and testing took the best part of a year. The first unit delivered in June, 1969 was the DEC PDP-14 which was installed to control a gear grinding machine. Later in the summer, 3-I delivered their unit designated the PDQ-II. PDQ stood for Program Data Quantizer and the 3-I unit was installed on the Forward Clutch Line assembly. Last in was Bedford Associates, now known as Modicon. Delivered in November, their unit was Bedford’s 84th project of 1969 and so carried the designation the 084. Like the DEC unit, it was installed to replace a relay panel controlling a gear grinding machine.

The Performance

The prototype controllers performed well and interest was spreading both inside General Motors and to automation supplier and manufacturing engineering services companies.

With continued operating experience, the Modicon 084 became the controller of choice for factory engineers and electricians. First and foremost, Modicon programmed in ladder logic, whereas the PDQ-II and PDP-14 programs were written in Boolean. This was fine for computer engineers but average plant engineers and electricians understood ladder and easily adapted to the 084. Other PLC builders soon emulated Modicon and offered ladder logic.
084 programming was relatively straight forward. The user plugged in a programming unit, selected an appropriate software module and keyed in ladder diagrams. Modicon also established a phone service for users programming and troubleshooting support. In an early interview based on a Bedford machine tool project, Dick Morley stated that their new PLC had reduced programming time from 6 months to 6 days.
In contrast, to re-program the PDQ-II, a Boolean program was written to punch a paper tape from a teletypewriter using a minicomputer interface. A special loader was used to load the program.
The PDP-14 also required a Boolean program on punched tape and used a woven wire rope circuit board memory. new program, tape and memory board was sent back to DEC for the program change, with about a one week turn-around.
The PDQ-II and PDP-14 were replaced in 1971. The 084 remained in service for more than 10 years. Hydra-matic eventually returned the initial 084 to Modicon and it has been on display at the Smithsonian.

Summary

By 1972, there were about a dozen PLC suppliers offering some 20 models. Both GE (Logitrol) and Square D (SY/MAX) developed their own PLC products and became major suppliers. Industrial Solid State Controls (ISSC) introduced its IPC-4000 and Honeywell became the sales agent. Reliance Electric introduced the Automate 33. Allen-Bradley. which bought out (Information Instruments, Inc), brought out the PMC (programmable matrix controller) and the PLC-1. Cutler Hammer began marketing an Entrekin-developed CON-64, and Datrak became a PLC supplier.

Who should be credited with inventing the PLC? Time-wise, Dick Morley’s January 1, 1968 memo is the earliest PLC documentation uncovered. In addition, Morley Patent '3,761,893' predates any other PLC patents. Even if Modicon hadn’t been the first to build a PLC, the company would still have a claim to being originators, as their model was the only one of the first three installed in a factory to survive. In addition, their 084 initiated features that became industry standards.