Industrial control programming and protocols on a Raspberry Pi
Modbus Writes from Node-RED
I modified the earlier ladder logic program to light the LED from either the pushbutton or a holding register (%QW1; Figure 12), which is an integer, so the value is converted to a boolean then OR'd with the pushbutton interface. The result of this OR is the value of the LED.
Figure 12: Ladder logic to light LED from either a pushbutton or Node-RED.
On Node-RED, a slider node is used to pass a 0 or 1 to a modbus tcp output node, which is configured as a single write to holding register 1. After the Node-RED logic is deployed, the web dashboard is accessed at: http://<your_rasp_pi>:1880/ui/ (Figure 13).
Figure 13: Node-RED dashboard with read/write access to OpenPLC variables.
Final Comments
Learning industrial control theory can be a little challenging: It's a very large topic with specific standards (e.g., IEC 61131-3 programming) and industry-specific communications packages like Modbus. Luckily, open source packages like OpenPLC allow you to familiarize yourself with industrial controls on low-cost hardware like the Raspberry Pi.
OpenPLC is an excellent testing and teaching tool, but it's important to point out that OpenPLC is not designed to be used on real-time projects that have environmental or safety concerns.
Infos
- OpenPLC documentation: https://www.openplcproject.com/
- IEC 61131-3 documentation: https://en.wikipedia.org/wiki/IEC_61131-3
- Node-RED documentation: https://nodered.org/
- OpenPLC install: https://www.openplcproject.com/getting-started/
- Modbus documentation: https://modbus.org/
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