Today’s story is about a new solution from Analog Device’s Circuits from the Lab® (CFTL) program. For over a dozen years, these lab-tested reference circuits have been used by designers and applications engineers to speed their time to market with solutions to complex problems. The challenge is best exemplified with a look at conditions on a typical factory floor - electrically noisy places packed with assembly lines, robotic devices and other electromechanical equipment. Often running 24/7, factory managers need highly reliable monitoring and diagnostic systems which can detect – and prevent – issues slowing or shutting down the line. What they need is something called a Programmable Logic Controller (PLC).

A PLC is basically a computer, but one which has been beefed up - engineers use the word “ruggedized” – to operate in electrically unfriendly manufacturing environments. In this Circuit from the Lab, ADI has taken four circuit boards (three of which are existing CFTLs) and stacked them together to make a small but complete PLC.

CM0435 Stack of boards

I looked at it and immediately thought of a four-decker sandwich which, it turns out, is not an inappropriate analogy. Each of the four layers of this stacked circuit sandwich has one or more specific functions which, when combined, make for a highly reliable PLC. Let’s deconstruct this sandwich and examine the layers:

  1. Because most factories will need more than one PLC the first board (itself a CFTL) uses a signaling standard known as RS-485 to provide a robust, reliable connection up to 256 PLCs, over distances up to a kilometer away.
  2. On the second board (also a CFTL) are four digital-to-analog converters (DAC). Their job is to take the digital instructions from the processor and turn them into an analog signal to control the equipment (motors don’t understand digital streams of ones and zeros, however they will respond to increases or decreases in voltage or current to tell them what to do).
  3. On the third board (another stand-alone CFTL) are four high-resolution analog-to-digital converters (ADC). An ADC receives data from factory equipment, which is an analog or real-world signal, and converts it to a digital data stream.
  4. The fourth and final board contains a powerful and flexible ADI processor. Its primary job is to maintain communication between the other boards and other PLCs on the BUS, so they all know what the other is doing at any given time. By using an industry standard communications protocol (they call it Modbus) this PLC can not only “talk” to the three other boards, but also legacy systems around the factory.

All four boards also have built-in protection from the common problem of electrical grounds of factory devices being at a different potential than the PLC. If you’ve ever shuffled your feet on a rug in the winter and then touched a doorknob you’ve felt the pain of different potential. In a factory environment those different potentials can wreak havoc by disabling the PLC or the equipment being measured or controlled.

Designing an isolation solution from scratch can take a long time. All the boards in ADI’s PLC solution are built with Isolation protection to protect them from potential disaster, saving customers a ton of time. In addition, each layer of Analog Devices’ PLC “sandwich” also comes with operational code and device drivers (more heavy lifting already done for you).

 To learn more about the CN0435 complete PLC solution, visit the ADI website. There you will find details on the circuit and the many ways it can be easily implemented for factory, healthcare and environmental monitoring as well as dozens of other applications. One can also order an evaluation kit containing all the parts of the sandwich. I mean PLC.

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