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Peek inside an industrial PLC

23 Jul 2015  | Steve Taranovich

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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 400MHz, 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 32bit 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 32bit 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, " 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 (128 K 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 Micro800 user-selectable modules. 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 towards 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.

About the author
Steve Taranovich is a senior technical editor at EDN with 41 years of experience in the electronics industry. Steve received his MSEE from Polytechnic University, Brooklyn, New York, and his BEEE from New York University, Bronx, New York. He is also chairman of the Educational Activities Committee for IEEE Long Island. His expertise is in analogue, RF and power management with a diverse embedded processing education as it relates to analogue design from his years at Burr-Brown and Texas Instruments. Steve was a circuit design engineer for his first 16 years in electronics. He then served as one of the first field application engineers with Burr-Brown Corp and also became one of their first global account managers, traveling to Europe, India and China.

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