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libmodbus

A featureful and portable Open Source Modbus library.

Description

libmodbus is a library to send/receive data with a device which respects the Modbus protocol. This library contains various backends to communicate over different networks (eg. serial in RTU mode or Ethernet in TCP IPv4/IPv6). The http://www.modbus.org site provides documentation about the Modbus Specifications and Implementation Guides.

libmodbus provides an abstraction of the lower communication layers and offers the same API on all supported platforms.

This documentation presents an overview of libmodbus concepts, describes how libmodbus abstracts Modbus communication with different hardware and platforms and provides a reference manual for the functions provided by the libmodbus library.

Use cases

The library can be used to write a:

  • client, the application reads/writes data from various devices.
  • server, the application provides data to several clients.

A libmodbus client that reads only the temperatures from sensors.

A libmodbus server that exposes data to a Grafana service.

Contexts

The Modbus protocol supports several transport protocols (eg. serial RTU, Ethernet TCP) called backends in libmodbus.

The first step is to allocate and set a modbus_t context according to the required backend (RTU or TCP) with a dedicated function, such as modbus_new_rtu. The function will return an opaque structure called modbus_t containing all necessary information to establish a connection with other Modbus devices according to the selected backend.

Once this context has been created, you can use use the common API provided by libmodbus to read/write or set the various timeouts. With this common API, it's easy to switch the backend of your application from RTU to TCP IPv6 for example.

RTU Context

The RTU backend (Remote Terminal Unit) is used in serial communication and makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. Modbus RTU is the most common implementation available for Modbus. A Modbus RTU message must be transmitted continuously without inter-character hesitations (extract from Wikipedia, Modbus as of Mar. 13, 2011, 20:51 GMT).

The Modbus RTU framing calls a slave, a device/service which handle Modbus requests, and a master, a client which send requests. The communication is always initiated by the master.

Many Modbus devices can be connected together on the same physical link so before sending a message, you must set the slave (receiver) with modbus_set_slave. If you're running a slave, its slave number will be used to filter received messages.

The libmodbus implementation of RTU isn't time based as stated in original Modbus specification, instead all bytes are sent as fast as possible and a response or an indication is considered complete when all expected characters have been received. This implementation offers very fast communication but you must take care to set a response timeout of slaves less than response timeout of master (ortherwise other slaves may ignore master requests when one of the slave is not responding).

To create a Modbus RTU context, you should use modbus_new_rtu.

You can tweak the serial mode with the following functions:

TCP (IPv4) Context

The TCP backend implements a Modbus variant used for communications over TCP/IPv4 networks. It does not require a checksum calculation as lower layer takes care of the same.

To create a Modbus TCP context, you should use modbus_new_tcp.

TCP PI (IPv4 and IPv6) Context

The TCP PI (Protocol Independent) backend implements a Modbus variant used for communications over TCP IPv4 and IPv6 networks. It does not require a checksum calculation as lower layer takes care of the same.

Contrary to the TCP IPv4 only backend, the TCP PI backend offers hostname resolution but it consumes about 1Kb of additional memory.

Create a Modbus TCP PI context, you should use modbus_new_tcp_pi.

Connection

The following functions are provided to establish and close a connection with Modbus devices:

In RTU, you should define the slave ID of your client with modbus_set_slave.

To analyse the exchanged data, you can enable the debug mode with modbus_set_debug.

Once you have completed the communication or at the end of your program, you should free the resources with the common function, modbus_free

Reads and writes from the client

The Modbus protocol defines different data types and functions to read and write them from/to remote devices. The following functions are used by the clients to send Modbus requests:

To read data:

To write data:

To write and read data in a single operation:

To send and receive low-level requests:

To reply to an exception:

Handling requests from server

The server is waiting for request from clients and must answer when it is concerned by the request. The libmodbus offers the following functions to handle requests:

Data mapping:

Receive:

Reply:

Advanced functions

Timeout settings:

Error recovery mode:

Setter/getter of internal socket:

Information about header:

Data handling

Macros for data manipulation:

  • MODBUS_GET_HIGH_BYTE(data), extracts the high byte from a byte
  • MODBUS_GET_LOW_BYTE(data), extracts the low byte from a byte
  • MODBUS_GET_INT64_FROM_INT16(tab_int16, index), builds an int64 from the four first int16 starting at tab_int16[index]
  • MODBUS_GET_INT32_FROM_INT16(tab_int16, index), builds an int32 from the two first int16 starting at tab_int16[index]
  • MODBUS_GET_INT16_FROM_INT8(tab_int8, index), builds an int16 from the two first int8 starting at tab_int8[index]
  • MODBUS_SET_INT16_TO_INT8(tab_int8, index, value), set an int16 value into the two first bytes starting at tab_int8[index]
  • MODBUS_SET_INT32_TO_INT16(tab_int16, index, value), set an int32 value into the two first int16 starting at tab_int16[index]
  • MODBUS_SET_INT64_TO_INT16(tab_int16, index, value), set an int64 value into the four first int16 starting at tab_int16[index]

Handling of bits and bytes:

Set or get float numbers:

Error handling

The libmodbus functions handle errors using the standard conventions found on POSIX systems. Generally, this means that upon failure a libmodbus function shall return either a NULL value (if returning a pointer) or a negative value (if returning an integer), and the actual error code shall be stored in the errno variable.

The modbus_strerror() function is provided to translate libmodbus-specific error codes into error message strings; for details refer to modbus_strerror.

Miscellaneous

The _LIBMODBUS_VERSION_STRING_ constant indicates the libmodbus version the program has been compiled against. The variables 'libmodbus_version_major', 'libmodbus_version_minor', 'libmodbus_version_micro' give the version the program is linked against.

Copying

Free use of this software is granted under the terms of the GNU Lesser General Public License (LGPL v2.1+). For details see the file COPYING.LESSER included with the libmodbus distribution.