libmodbus

A Modbus library for Linux, Mac OS X, FreeBSD, QNX and Win32

libmodbus is a free software library to send/receive data according to the Modbus protocol. This library is written in C and supports RTU (serial) and TCP (Ethernet) communications.

The license of libmodbus is LGPL v2.1+ and the licence of programs in the tests directory is BSD 3-clause.

The documentation is provided with libmodbus releases as man pages (generated from doc sources). You can also browse the documentation of the different releases of libmodbus:

See the Wiki for general informations.

Code Sample

  modbus_t *mb;
  uint16_t tab_reg[32];

  mb = modbus_new_tcp("127.0.0.1", 1502);
  modbus_connect(mb);

  /* Read 5 registers from the address 0 */
  modbus_read_registers(mb, 0, 5, tab_reg);

  modbus_close(mb);
  modbus_free(mb);

External Documentation

Modbus Specifications and Implementation Guides, this page contains links to the following documents:
- MODBUS Protocol Specification (Modbus_Application_Protocol_V1_1b.pdf)
- Modbus Serial Line Protocol and Implementation Guide V1.02 (Modbus_over_serial_line_V1_02.pdf)
- MODBUS Over Serial Line FOR LEGACY APPLICATIONS ONLY (PI_MBUS_300.pdf)
- MODBUS TCP/IP (Modbus_Messaging_Implementation_Guide_V1_0b.pdf)
- Object Messaging Specification for the Modbus TCP Protocol (Object_Messaging_Protocol_ExtensionsVers1.1.doc)
Wikipedia page of Modbus