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).
As 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.
An Allen-Bradley product video is at; https://youtu.be/RrJRoge4p30
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.
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.
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.
Optocouplers have been the go-to isolation technology for many years, though more recently, digital isolators from the likes of Silicon Labs, Texas Instruments, and Analog Devices, have been promoted as an inexpensive and flexible smaller-form-factor alternative. Other, more-traditional small-form-factor options include magnetic and capacitive isolation. Working in favour of digital isolators and other options is the perception that optocouplers may be unreliable over time as the LED’s efficiency decreases, causing the output to become unstable.
With all that said, then, it induced a state of cognitive dissonance to open up the Micro850 and see optocouplers as the sole isolation mechanism in one of the most environmentally demanding and long-life applications of electronic I/O – an industrial PLC.
Specifically, the DC inputs are isolated using six Avago Technologies 10 MBaud (high-speed) HCPL-063L 2-channel optocouplers and four standard, lower-speed Toshiba TLP280-4 4-channel photocouplers with bit rates in the 10-kBaud to 100-kBaud range.
To understand why Rockwell Automation chose optocouplers for the input and output isolation of its PLCs, it helps to understand the application requirements and what optocouplers bring to the table.
For starters, voltage transients and glitches are a widespread problem in industrial environments. Common sources include field devices such as sensors and motor starters. These can be sudden, fast and of sufficiently high intensity to blow the drive circuitry, control logic and even hurt the operator, so it’s very much a safety issue (Figure 3a)
Figure 3. In industrial applications, isolation is required to block voltage transients that could harm logic circuits or the operator (a) as well as to eliminate common-mode transients that can cause signal errors (b).
Also in PLCs, the different isolated grounds used between circuitries can create a common-mode problem (Figure 3b). This is more of a signal-integrity issue, versus safety, as the result is unreliable signalling or poor noise rejection.
Two more sources of circuit problems are ground loops and situations where circuits operate at different voltage levels (Figure 4).
Figure 4. When done well, isolation can serve to avoid ground loops caused by a difference in potential between grounding points in a system.
In ground loops, unwanted currents flow between two points that share a common path in an electrical system but have finite and differing impedances at each point’s ground potential level. Where two circuits operate closely yet at dangerously variant voltage levels, high-voltage safety isolation is needed, and may also be needed where level shifting is used, causing circuit incompatibility at different voltage levels.
Why choose optocouplers?
The reasons are many. For starters, they provide true galvanic isolation, with a high level of insulation between the LED and the detector using silicone and Kapton tape with 0.08 mm of clearance (Figure 5).
Figure 5. Optocouplers provide galvanic isolation that meets or surpasses regulatory requirements such as IEC 60747-5-5.
With galvanic isolation, electrical charge-carrying particles cannot move from one electric circuit to another, though signal information is still exchangeable between circuits by other means, such as optically, inductive coupling, or capacitive.
The safety isolation regulatory compliance issue is something to look out for as a designer. IEC 60747-5-5 is the component level safety standard for optical isolation in industrial applications that cannot be matched by equipment safety standards. Many technologies will “meet” the base standards, but optocouplers can exceed them to provide some valuable headroom for extreme situations.
Note that IEC 60747-5-5 is the de facto industry standard for optocouplers certified for Reinforced Isolation. If a single layer of insulation has the same insulation property as double insulation, this is known as Reinforced Insulation. While the Avago devices in the Micro850 meet voltage-withstand ratings of 2.5 to 3.75 kV, optocouplers can go further and reach 7.5 kV.
Despite being suspected of having unacceptable degradation over time, as mentioned earlier, optocouplers have actually been proven to have a long lifespan, with little noticeable degradation over 30 years (Figure 6).
Figure 6. Well-made optocouplers with photo-IC outputs have been shown to have lifespans past 30 years, which puts to rest rumours that they suffer from degradation over the useful lifespan of a system.
Such concerns may be valid for earlier or relatively inexpensive phototransistors, but do not apply to a high-performance optocoupler with photo-IC outputs.
Figure 7a. Input isolation characteristics of HCPL-063L optocoupler technology used on the Micro850 digital I/O board, compared to a newer version, the ACPL-064L, showing much lower drive current and power consumption.
Figure 7b. Output isolation characteristics of the HCPL-M456 optocoupler used in the Micro850 compared to the newer version of that device, the ACPL-M483, showing lower LED drive current, faster propagation and improved common-mode rejection (CMR).
Another advantage of optocouplers, such as the HCPL-063L and its updated version, the low LED drive current ACPL-064L (Figure 7a) is that they don’t need a supply voltage. While alternative isolation devices have the advantage of channel density – Silicon Labs’ Si86xx line offers six channels in a compact 16-pin SOIC – optocouplers don’t need additional isolated power supply at the field-device inputs side. That said, they do require some minor calculations, in order to choose appropriate input split resistors to control the LED drive current.
Given the environment in which PLCs are used, it’s worth noting that optocouplers are also immune to electromagnetic interference (EMI), another good reason Rockwell would choose them.
As shown in Figures 7a and 7b, the state of the art for optocouplers continues to advance with higher efficiency (i.e. ever-lower LED drive current), higher operating temperatures, and smaller form factors. Insulation and EMI are likely to become increasingly important as regulatory standards continue become more stringent.
Accompanying the optocouplers on the I/O board are a Lattice LC4064V 400-MHz, 64-macrocell complex programmable logic device (CPLD) to knit the functions together, and some ON Semiconductor power devices around the power supply.
PLC core processing functions
The main logic and processing board is where PLCs compete with the PC/embedded computer with regard to functionality, programmability, and the user interface. In the case of the Micro850, the core decision-making, system management, runtime control and and user-interface processing is performed by a Freescale (now NXP) Coldfire MCF5372 32-bit ROM-less MCU. The MCU would also be running the Connected Components Workbench software to program and reprogram the PLC as needed.
Figure 8. The main processing board on the Micro850 comprises a Freescale (now NXP) Coldfire MCF5372 32-bit MCU for system management, runtime control and the user interface, as well as a Xilinx Spartan XC351400A FPGA for communications and motor control, and likely for some proprietary functionality as well.
The Xilinx Spartan XC3S1400A FPGA is most likely dedicated to proprietary logic functions, the high-speed communications control, as well as to lower total cost of ownership by, “…by conserving power through tighter control of speed, torque, and acceleration, while improved efficiency allows for smaller, less expensive motors.” (See “Using FPGAs to solve challenges in industrial applications.”)
The Spartan FPGA would be at the core of the Micro850’s ability to perform motion control, supporting and taking advantage of as many as three axes with pulse-train outputs (PTOs), per the PLC’s description.
The FPGA and the MCU are supported by Cypress Semiconductor’s FM21L16, 2-Mbit (128k x 16) FRAM memory (drop-in replacement is now the FM28V202A), a Micron Technology MT48LC8M 128 Mbit (8 Mbit x 16) SDRAM, and Analog Devices ADM3315EA 15-kV, ESD-protected serial-port transceiver w/Green Idle. Voltage monitoring is provided by an Analog Devices ADM706 3-V supervisory circuit.
Figure 9. The back of the main processing board for the Micro850 shows the main user I/O as well as the five interfaces for Micro800 add-on modules that can expand the PLC’s functionality as needed, making it extremely flexible.
The back of the main processing board shows the Ethernet, non-isolated serial, and USB programming ports, as well as the five connectors for the . The modules enable a flexible mix of I/O and communications as the applications’ requirements change over time and PLC needs to adapt to newer technology.
It’s this flexibility, combined with more powerful processing, programming simplicity, and ruggedness that keep PLCs at the forefront of industrial control platforms.
The result is that over time the decision between PLC and PC/embedded-computer-based systems has become more to do with installed base, designer familiarity – which can affect project development time – 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.