ModbusMaster | Enlighten your Arduino to be a Modbus master

You are using the wrong Modbus function. You should be using function 0x03 readHoldingRegisters (note the 0x03 at the beginning of the frame on the bottom side of the page).

Just change this line:

At runtime, the code executed for a Java program starts out in "class files", stored by the compiler in files with a ".class" extension. These files are often packaged in groups in "jar files", and the Java runtime can find the classes in the jar files while it's running.

Your error indicates that the runtime cannot find the class indicated. The java.io.SerialPortEventListener class appears to be available in the jar file rxtx-2.1.7.jar. You can download that (search for its reliable source) and copy it to the target machine. You'll then need to read up on what a classpath is; you can set a classpath in an environment variable or with a parameter passed to the "java" command at runtime. Either way, the runtime can find the class in that jar file when you run your program.

I've touched on a handful of concepts here; am sorry it isn't a real simple do 1-2-3 kind of answer, but you'll need to understand jar files and the classpath as you code more.

Many people fail to realise, that communications take time. So, in your case, you open the device and immediatelly initiate a read request, which fails, because the device is not open yet (because it takes time, as everithing else).

In a GUI application, it is probably the best, when the code reacts to events, instead of trying to execute everything lineary. This is where signals and slots come into play.

Consider the following workflow:

when you create the QModbusDevice, you also connect its signals (e.g. stateChanged and errorOccurred) to slots in your code (e.g. MainWindow::onModbusStateChanged, or MainWindow::onModbusErrorOccurred). Those should be considered as callback functions, which are executed when the corresponding event occurs: the connection state changes, the data is received, the data is sent, an error occurs.

In this manner you would know exactly when to do what.

Here is an example code I have prepared for you to show you how the problem could be approached:

in MainWindow.h

You need to be careful with your data types.

First thing to consider is that one Modbus register stores 16 bit.

So two registers should be 32 bit and 4 registers 64 bit.

With that in mind, you need to review the manual of your device and see the width of each data word, which is the number of registers you need to read.

It depends on how your data is represented. In Modbus you need two registers to store a float. Then you need to check on the manual of your device the endianness of these registers and the order of the bits within.

If you read registers two at a time then you can use the helper function provided with phpModbus:

It depends on type of an equipment, you are working with. You just have to follow the logic, described in equipment mans for this particular exception.

In general there is no special 'Program Complete' event. That means, as it is written for 0x5 - "Specialized use in conjunction with programming commands.". So you just have to poll (read) some flag from your device meaning the internal process in device, which caused this exception, is complete.

Just as an example, I've met with such exception in relay protection device, which issued it when it has been in a process of writing a disturbance record. I had just to check for that record readiness in some time.