PLCs have a long and storied history, with Allen-Bradley itself coining the term “programmable logic controller” in 1971 when it introduced its version of what was then called the “programmable controller.” For anyone who cut their teeth on ladder logic can testify, PLCs at the time were an elegantly simple solution to an age-old problem: making control systems reconfigurable without having to manually rewire or reconnect the hardware. The modern programmable logic controller (PLC) is still today very important part of factory, industrial, and manufacturing automation. It’s this flexibility, combined with more powerful processing, programming simplicity, and ruggedness that keep PLCs at the forefront of industrial control platforms. Over time the decision between PLC and PC/embedded-computer-based systems has become more to do with installed base, designer familiarity and legacy perceptions, than real technological or ruggedness differentiators. As we move toward the Internet of Things (IoT) and Industry 4.0 or 4.2, the choice may sway back and forth again for new factories and systems, but odds are the two architectures will coexist for many years to come.
Teardown: Ruggedness and flexibility keep PLCs strong in industrial article shows a tear down a popular PLC, the Allen-Bradley Micro850. The article explore some of the choices made in its design to shed light on core I/O isolation options along with some of the elements that go into a well-known PLC design.
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Tomi Engdahl says:
Teardown: Ruggedness and Flexibility Keep PLCs Strong in Industrial
http://www.eetimes.com/document.asp?doc_id=1327041
The modern programmable logic controller (PLC) is at the nexus of two debates that are taking place daily at opposite ends of the control-system spectrum. At one end is the debate over the ideal technology for digital I/O isolation and protection. At the other end, and at a much higher architectural level, is the debate over which is better: PLC-based control or PC/embedded computer-based control.
Given the increasing importance of factory, industrial, and manufacturing automation, we jumped on the opportunity to tear down a popular PLC, the Allen-Bradley Micro850, and explore some of the choices made in its design to shed light on core I/O isolation options along with some of the elements that go into a well-known PLC design.
For industrial control and automation, these Windows-based PCs and embedded computers offered higher processing power, greater programming flexibility, more ecosystem support and lower cost.
Meanwhile, PLCs held on to their core advantages of ruggedness, simplicity, reliability, durability and “trust,” a critical factor when downtime can result in losses ranging from thousands to many millions of dollars. Control engineers and technicians knew they could rely upon PLCs and knew how to troubleshoot or swap them out quickly and easily if anything ever did go wrong.
While PCs may have been invading the factory floor, PLCs weren’t standing still.
Like most PLCs, the Micro850 is designed for standalone operation, but is easily configured for custom applications and more I/O using its support for up to five Micro800 plug-in modules and up to four Micro850 expansion I/O modules, for up to 132 I/O points. It operates over the temperature range of -20 to 65°C (-4 to 149 °F).
Tomi Engdahl says:
https://hackaday.com/2024/02/15/a-1960s-plc-gives-up-its-secrets/
Tomi Engdahl says:
Hackaday Europe 2026: The 1-Bit CPU That Ran Factories
https://hackaday.com/2026/08/19/hackaday-europe-2026-the-1-bit-cpu-that-ran-factories/
Once upon a time, though, things weren’t so sophisticated. [Nicola Cimmino] came to Hackaday Europe 2026 to tell us all about a remarkably simple 1-bit CPU that used to run factories.
Eventually, one unremarkable looking chip caught his attention—the Motorola MC14500B. This chip was rather unique, being a rather simple processor with just 16 instructions and a 1-bit data bus.
It’s worth examining the era in which this chip existed. Intel dropped the 4-bit 4004 in 1971, with the famous 8-bit 8080 landing in 1974. The Zilog Z80 came along in 1976, similarly an 8-bit design. And yet, when Motorola released the MC14500 in 1977, it landed with a rather slimline 1-bit design instead. Nicola notes that this likely came down to price, since populating a chip with more transistors cost more money quite significantly back in the 1970s. If the job could be done with less, it would make the part cheaper and thus more popular in the market. Bearing this out, Nicola explains that a 1976 Zilog Z80 used 8,500 transistors and cost around $200 USD, while an MC14500 used just 500 transistors and could be had in 1977 for the bargain price of just $5 USD.
Back in the mid-1970s, automation in industry often consisted of simple logic that was handled by cabinets full of relays.
Changing logic required manually rewiring things which could be fussy and tedious at the best of times. In those days, the Programmable Logic Controller was just coming into use, developed to be a reprogrammable system for industrial automation tasks that was more flexible and reconfigurable just by reprogramming it.
The MC14500 sprung up as a useful tool at this time, powering a great many programable industrial systems. It was designed to offer the bare minimum requirements for its application, while leaving extraneous hardware for designers to implement if and when it was needed
This setup would only allow for doing combinational logic, since there is nowhere to store the current state of the system. However, hooking some outputs back to the inputs could allow for sequential logic, since it would allow for storing the current state of the system via those outputs.