FieldTalk^™ Modbus^® Master Protocol Library
C++ Editions

How to integrate the Protocol in your Application

Using Serial Protocols

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. Include the package header files

#include "MbusRtuMasterProtocol.hpp"

2. Device data profile definition

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

short readRegSet[20];
short writeRegSet[20];
int readBitSet[20];
int writeBitSet[20];

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

float readFloatSet[10];
float writeFloatSet[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:

long readLongSet[10];
long writeLongSet[10];

Note that because a long 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 mbusProtocol;

4. Open the protocol

   int result;

   result = mbusProtocol.openProtocol(portName,
                                      9600L, // Baudrate
                                      8,     // Databits
                                      1,     // Stopbits
                                      0);    // Parity
   if (result != FTALK_SUCCESS)
   {
      fprintf(stderr, "Error opening protocol: %s!\n",
                       getBusProtocolErrorText(result));
      exit(EXIT_FAILURE);
   }

5. Perform the data transfer functions

To read register values:

mbusProtocol.readMultipleRegisters(1, 100, readRegSet, sizeof(readRegSet) / sizeof(short));

To write a single register value:

mbusProtocol.writeSingleRegister(1, 200, 1234);

To write mutliple register values:

mbusProtocol.writeMultipleRegisters(1, 200, writeRegSet, sizeof(writeRegSet) / sizeof(short));

To read discrete values:

mbusProtocol.readCoils(1, 10, readBitSet, sizeof(readBitSet) / sizeof(int));

To write a single discrete value:
```
mbusProtocol.writeCoil(1, 20, 1);
```

To write multiple discrete values:

mbusProtocol.forceMultipleCoils(1, 20, sizeof(writeBitSet) / sizeof(int));

To read float values:

mbusProtocol.readMultipleFloats(1, 100, readFloatSet, sizeof(readFloatSet) / sizeof(float));

To read long integer values:

mbusProtocol.readMultipleLongInts(1, 100, readLongSet, sizeof(readLongSet) / sizeof(long));

6. Close the protocol port if not needed any more

mbusProtocol.closeProtocol();

7. Error Handling

Serial protocol errors like slave device failures, transmission failures, checksum errors and time-outs return an error code. The following code snippet can handle and report these errors:

   int result;

   result = mbusProtocol.readMultipleRegisters(1, 100, dataSetArray, 10);
   if (result != FTALK_SUCCESS)
   {
      fprintf(stderr, "%s!\n", getBusProtocolErrorText(result));
      // Stop for fatal errors
      if (!(result & FTALK_BUS_PROTOCOL_ERROR_CLASS))
         return;
   }

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

Using MODBUS/TCP Protocol

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