For a recent project, I had an interesting challenge. It involved a (relatively) simple control system, with an Allen Bradley MicroLogix PLC, and 30 or so inputs and outputs. The challenge was that the customer wanted to be able to access (and potentially control) live machine data from anywhere on their network, while buying as little hardware or software as possible. After a bit of research I came up with the following solution:
The only entryway to the data that was available with no additional hardware was the Modbus RTU network already in place, that fed data from the PLC (master) to some intelligent analog gauges (slaves) in the field. If I tapped into that, and sent the signal through a cheap Modbus RTU to TCP/IP converter (the only hardware addition necessary), I knew I could just plug into the customer’s existing Ethernet network, and have all the data I needed available from any Modbus TCP/IP server also plugged into the network.
That was the easy part. The hard part was finding a way to display that data in a meaningful way on a PC. After a bit of research I found two options. Two libraries, one for Visual Studio .NET offered by Win-TECH, and one for Java (called jamod), that could set up both client and server Modbus TCP/IP connections from a PC. The core differences were that the .NET library was supplied by a private company and cost thousands of dollars, whereas jamod was open source, had an active online community, and was free to distribute under a modified BSD license. Even after weighing the risks of leaning on an open source solution (lack of support), the jamod solution seemed a little more enticing.
After surprisingly little tinkering around, I had it up and running. The library simply extends the regular Java TCP/IP Connections class, and offers many of the same connectivity functions, along with all of the standard Modbus communication functions. It really opens up the possibilities for custom remote monitoring software. My customer can now log into their machine, check the status of (or modify) all of its parameters, and log the data to a local CSV file, all from any computer that can run the Java Runtime Environment. Even neater, the JRE is available on many mobile platforms, giving anyone with the right smart phone the ability to peek into the machine data.
Jamod truly opens some exciting doors. This open source library, combined with Modbus (the most widely used protocol in the manufacturing world), makes it easier than ever to create customized remote monitoring and control applications with extremely low hardware and software costs.
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