How to Write to Modbus TCP Registers Using C Language

Modbus is a widely-used communication protocol in industrial automation and data acquisition systems. Modbus TCP, the TCP/IP-based version of Modbus, allows for reliable communication over Ethernet networks. This article will guide you through the process of writing to Modbus TCP registers using the C language, with examples using popular Modbus libraries such as libmodbus and ModbusMaster.

Introduction to Modbus TCP and C Language

Modbus is a serial communication protocol that has been widely adopted for its simplicity and reliability. Modbus TCP extends this protocol to work over TCP/IP networks, making it suitable for modern networked systems. The C language allows for powerful and efficient programming, making it a popular choice for interacting with hardware and network devices.

Using libmodbus Library

The libmodbus library provides a powerful and flexible way to interact with Modbus devices from within a C program. Here is an example of how to write to a Modbus TCP register using libmodbus:

#include modbus.hint main() {    modbus_t *ctx;    uint16_t data[1]  {0001}; // Data to write to register    int rc;    ctx  modbus_new_tcp("192.168.1.10", 502); // Connect to Modbus TCP slave at IP address 192.168.1.10 on port 502    if (ctx  NULL) {        fprintf(stderr, "Failed to create the context
");        return 1;    }    modbus_set_slave(ctx, 1); // Set slave address    rc  modbus_connect(ctx); // Connect to slave    if (rc  -1) {        fprintf(stderr, "Connection failed
");        modbus_free(ctx);        return 1;    }    rc  modbus_write_register(ctx, 0, data[0]); // Write data to register 0    if (rc  -1) {        fprintf(stderr, "Failed to write to register
");        modbus_close(ctx);        modbus_free(ctx);        return 1;    }    modbus_close(ctx); // Close connection    modbus_free(ctx); // Free context    return 0;}

This code snippet demonstrates how to write the value 0001 to register 0 of a Modbus TCP slave with the IP address 192.168.1.10 and slave address 1.

Using ModbusMaster Library

ModbusMaster is another library that simplifies Modbus communication in C. Below is an example of how to write to a Modbus TCP register using this library:

#include ModbusMaster.hvoid setup() {    (9600); // Initialize serial communication    ("Modbus Master Library Example");    // Create a new Modbus node, set slave address to 1 and use Serial port for communication    Node node(1, Serial);    // Write value 1 to register 0    node.writeSingleRegister(0, 1);}void loop() {    node.poll(); // Handle Modbus communication}

This code snippet demonstrates how to write the value 1 to register 0 of a Modbus TCP slave with address 1 connected to a serial port.

Conclusion

To write to Modbus TCP registers using the C language, libraries such as libmodbus and ModbusMaster provide comprehensive tools. These examples demonstrate the process of writing to a Modbus TCP register, which can be adapted to specific setups and requirements.

Keywords: Modbus TCP, C language, libmodbus library