How do I connect to device using jamod and interpret the data

Question

My client wants to control the HVAC systems installed in their site with a custom solution. The HVAC devices provide MODBUS TCP/IP connectivity. I'm new to this field and have no knowledge of MODBUS. I searched the internet and found jamod as a java library for MODBUS. Now I would like to write a program using jamod. But my confusion is how do I get the address of the device I want to connect. And my second problem is even if I manage to connect the device , how can I get required data (in engineering units like temperature) from MODBUS. My questions may sound awful but please forgive me as I'm a novice in this field.

Answer 1

How do I get the address of the device I want to connect to?

This kind of depends on if you're connecting over Modbus RTU or Modbus TCP. RTU (serial) will have a slave id you'll specify while tcp is more direct and the slave id should always be 1.

How can I get required data (in engineering units like temperature) from MODBUS?

Hopefully the data is already formatted in engineering units. Check the device's manual and there should be a table or chart mapping registers to values.

Example:

String portname = "COM1"; //the name of the serial port to be used
int unitid = 1; //the unit identifier we will be talking to, see the first question
int ref = 0; //the reference, where to start reading from
int count = 0; //the count of IR's to read
int repeat = 1; //a loop for repeating the transaction

// setup the modbus master
ModbusCoupler.createModbusCoupler(null);
ModbusCoupler.getReference().setUnitID(1); <-- this is the master id and it doesn't really matter

// setup serial parameters
SerialParameters params = new SerialParameters();
params.setPortName(portname);
params.setBaudRate(9600);
params.setDatabits(8);
params.setParity("None");
params.setStopbits(1);
params.setEncoding("ascii");
params.setEcho(false);

// open the connection
con = new SerialConnection(params);
con.open();

// prepare a request
req = new ReadInputRegistersRequest(ref, count);
req.setUnitID(unitid); // <-- remember, this is the slave id from the first connection
req.setHeadless();

// prepare a transaction
trans = new ModbusSerialTransaction(con);
trans.setRequest(req);

// execute the transaction repeat times because serial connections arn't exactly trustworthy...
int k = 0;
do {
  trans.execute();
  res = (ReadInputRegistersResponse) trans.getResponse();
  for (int n = 0; n < res.getWordCount(); n++) {
    System.out.println("Word " + n + "=" + res.getRegisterValue(n));
  }
  k++;
} while (k < repeat);

// close the connection
con.close();  

Answer 2

First, "address" is ambiguous when you're working with Modbus/TCP since there is the IP address of the slave, the unit number of the thing you're talking to (typically 0 for Modbus/TCP), and the address of any registers.

For the "engineering units" question, what you're going to want is the Modbus register map, with any units or conversion factors included. You may also need to know data types, since all Modbus registers are 16 bits.