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PLC teardown: Allen-Bradley Micro850 for industrial apps

Posted: 14 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Rockwell Automation? PLC? programmable logic controller? embedded? IoT?

Another advantage of optocouplers, such as the HCPL-063L and its updated version, the low LED drive current ACPL-064L 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.

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 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 programme and reprogramme the PLC as needed.

Micro850

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, "...by conserving power through tighter control of speed, torque, and acceleration, while improved efficiency allows for smaller, less expensive motors."

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.

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