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).