Industrial control programming and protocols on a Raspberry Pi

Structured Text

One of the advantages of FBDs is that they are very readable and somewhat self-documenting. The downside of FBDs is that they can be messy for complex conditional logic. Structured text (ST) was developed as a programming option that can work along with the other 61131-3 languages. Structured text is block-structured and resembles Pascal syntactically.

For my ST program, I wanted to create the same functionality as in the earlier FBD. Interestingly, the same functionality in ST took only three lines of code (Figure 8), compared with five lines in FBD. In my ST program I also added a simple IF condition to reset the pushbutton counter if the value reaches 1000.

Figure 8: Structured text to count pushbutton presses.

Note that library functions such as R_TRIG are available in all the 61131-3 programming languages, and you can create your own custom functions in one programming language that can be used in all the other languages.

Running OpenPLC Programs

To start the runtime application manually on the Raspberry Pi, enter:

$ cd OpenPLC_v3
~/OpenPLC_v3$ sudo ./ &

The OpenPLC runtime starts the web application on port 8080 on the Raspberry Pi.

After logging in to the web interface, the first step is to select the Hardware option and set the OpenPLC hardware layer to Raspberry Pi. Next, select the Programs option and upload the OpenPLC configuration file. After a new configuration file is uploaded and compiled, the final step is to press the Start PLC button.

For my PLC application, a button push lights the LED for 3 seconds and the function block and structured text counter variables increment up.

The Monitoring option can be used to view the status of variables in the PLC configuration. At this point, the PLC is working somewhat "headless," so adding a Node-RED visual interface is next.

Modbus with Node-RED

Modbus [5] was the earliest and most common communication protocol used to connect industrial devices together. Modbus can be used with serial interfaces (Modbus RTU) or on Ethernet networks (Modbus TCP); both are supported by OpenPLC.

Node-RED has a number of Modbus TCP nodes that can be used. I found that node-red-contrib-modbustcp worked well for my application. New nodes can be added to Node-RED from the Manage Palette option.

A simple Node-RED application that can monitor the LED and counter statuses would use three modbustcp input nodes, a text node, and two numeric dashboard nodes (Figure 9).

Figure 9: Modbus interface logic in Node-RED.

Modbus supports four object types: coils, discrete inputs, input registers, and holding registers (Figure 10). On this Node-RED application, I am only using two types of Modbus objects: a coil (a single-digit bit, LED) and holding registers (16 bits, counters). When Modbus reads a holding register, it returns just a single value (more, if requested); however, for a coil, Modbus returns 16 bits of information, not just the single bit of interest.

Figure 10: Modbus object types.

To show just the LED status on a Node-RED dashboard, a small function is needed (Only pass item 0) to change the message payload to just the first item in the array:

msg.payload = msg.payload[0];
return msg;

The modbustcp input node needs to be configured with the variable's Modbus object type and address. For example , the LED's IEC addressing is %QX0.2, which would be a coil at address 2 (Figure 11). The function block counter (CNT_FB) address, %QW100, is a holding register at address 100 (CNT_ST is a holding register at address 0).

Figure 11: Node-RED Modbus input configuration for the Pi LED.

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