Let's assume we want to talk to a Modbus slave device with slave address 1.

The registers for reading are in the reference range 4:00100 to 4:00119 and the registers for writing are in the range 4:00200 to 4:00219. The discretes for reading are in the reference range 0:00010 to 0:00019 and the discretes for writing are in the range 0:00020 to 0:00029.

1. Import the library package

import com.focus_sw.fieldtalk.*;

2. Device data profile definition

Define the data sets which reflects the slave's data profile by type and size:

short[] readRegSet = new short[20]
short[] writeRegSet = new short[20]
boolean[] readBitSet = new boolean[20]
boolean[] writeBitSet = new boolean[20]

If you are using floats instead of 16-bit shorts define:

float[] readRegSet = new float[10]
float[] writeRegSet = new float[10]

Note that because a float occupies two 16-bit registers the array size is half the size it would be for 16-bit shorts!

If you are using 32-bit ints instead of 16-bit shorts define:

int[] readRegSet = new int[10]
int[] writeRegSet = new int[10]

Note that because an int occupies two 16-bit registers the array size is half the size it would be for 16-bit shorts!

3. Declare and instantiate a protocol object

MbusRtuMasterProtocol mbus = new MbusRtuMasterProtocol ();

4. Open the protocol port

mbus.openPort("COM1", 9600, MbusRtuMasterProtocol.DATABITS_8,
                            MbusRtuMasterProtocol.STOPBITS_1,
                            MbusRtuMasterProtocol.PARITY_NONE);

5. Perform the data transfer functions

To read register values:

mbus.readMultipleRegisters(1, 100, readRegSet);

To write a single register value:

mbus.writeSingleRegister(1, 200, writeRegSet);

To write a single float value:

mbus.writeSingleRegister(1, 210, 3.141F);

To write mutliple register values:

mbus.writeMultipleRegisters(1, 200, writeRegSet);

To read boolean (discrete) values:
```
mbus.readCoils(1, 10, readBitSet);
```
To write a single boolean (discrete) value:
```
mbus.writeCoil(1, 20, true);
```

To write multiple boolean (discrete) values:

mbus.forceMultipleCoils(1, 20, writeBitSet);

6. Close the protocol port if not needed any more

mbus.closePort();

7. Error Handling

Serial protocol errors like slave device failures, transmission failures, checksum errors and time-outs throw appropriate exceptions, which are all derived from the IOException and BusProtocolException class. The following code snippet can handle these errors:

try
{
   mbus.readMultipleRegisters(1, 100, dataSetArray);
}
catch (BusProtocolException e)
{
   Has to be caught before IOException!
   Protocol error management, something is wrong
   with the slave device or the bus.
}
catch (IOException e)
{
   If this occurrs after we checked for a BusProtocolException
   then it signals that the local I/O subsytem failed, which is
   fatal.

}

An automatic retry mechanism is available and can be enabled with mbus.setRetryCnt(3) before opening the protocol port.

TCP/IP Protocols

Let's assume we want to talk to a Modbus slave device with unit address 1 and IP address 10.0.0.11.