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.
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.” Allen-Bradley was since bought by Rockwell Automation. The term PLC quickly took hold, especially as the personal computer (PC) emerged and took the PC acronym. (For more background on the PLC’s birth and evolution, see “History of PLC and DCS,” by Segovin and Theorin).
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. This programmability foundation would soon put it head to head with the PC and later embedded computers, as they ventured onto the factory floor.
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. PCs seemed to be winning the battle in the late nineties and 2000s, but PLCs were becoming more powerful and adopting more standard operating systems and programming languages and methodologies, such as C, while also becoming more open. Such is the case with the Micro850, the PLC we chose to teardown. It uses Connected Components Workbench software, based on proven Rockwell Automation and Microsoft Visual Studio technology.
The Connected Components Workbench is a visual interface that lowers cost and speeds development time through the use of user-defined function blocks, tag configuration and screen design.
Specifically, we tore down the Micro850 2080-LC50-48QBB, a 28- to 24-V DC/V AC input, 20- to 24-V DC source output controller costing around $500 from various suppliers (Figure 1).
Figure 1 The Allen-Bradley Micro850 programmable logic controller (PLC) costs around $500 yet is symbolic of the high level of ruggedness configurability, isolation performance and ease of use that keeps PLCs strong in factory automation despite the encroachment of PCs/embedded computing.
The base 48-point controller comes with 100-kHz high-speed counter (HSC) inputs, embedded communications via a USB programming port, a non-isolated serial port (for RS-232 and RS-485 communications) and an Ethernet port. It provides embedded motion-control capabilities by supporting as many as three axes with pulse-train outputs (PTOs), and communicates via EtherNet/IP.
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).
The Micro850’s flexibility, communications and I/O capabilities allow it to support a wide range of applications, including: conveyors, cutting, material handling, sorters, packaging, shrink sleeving machines, solar panel positioning, and vertical form, fill and seal.
Opening the Micro850 reveals the main digital I/O board, a good starting point for a discussion of the optimum input- and output-signal-isolation technology (Figure 2). The inputs connect to field devices such as proximity, pressure and temperature sensors or push buttons, while the outputs connect devices such as indicator lights, motor starters, and solenoid valves.
Figure 2 Rockwell chose optocouplers to form the heart of its isolation strategy for the device inputs and outputs on its digital I/O board for the Micro850 PLC. Click here for larger image.
These field devices typically operate in an electrically noisy environment and are subject to high transient voltage surges, crosstalk, interference and power glitches, so isolation is required to maintain effective communication between the field device and the I/O module’s controller and to prevent damage.