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LIBMODBUS(7)		       Libmodbus Manual			  LIBMODBUS(7)

NAME
       libmodbus - a fast and portable Modbus library

SYNOPSIS
       #include <modbus.h>

       cc `pkg-config --cflags --libs libmodbus` files

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/IPv6). The http://www.modbus.org site provides documentation
       about the protocol at http://www.modbus.org/specs.php.

       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.

   Contexts
       The Modbus protocol contains many variants (eg. serial RTU or Ehternet
       TCP), to ease the implementation of a variant, the library was designed
       to use a backend for each variant. The backends are also a convenient
       way to fulfill other requirements (eg. real-time operations). Each
       backend offers a specific function to create a new modbus_t context.
       The modbus_t context is an opaque structure containing all necessary
       information to establish a connection with others Modbus devices
       according to the selected variant.

       You can choose the best context for your needs among:

       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,
	   http://en.wikipedia.org/wiki/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(3). 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).

	   Create a Modbus RTU context
	       modbus_new_rtu(3)

	   Set the serial mode
	       modbus_rtu_get_serial_mode(3)modbus_rtu_set_serial_mode(3)modbus_rtu_get_rts(3)modbus_rtu_set_rts(3)

       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.

	   Create a Modbus TCP context
	       modbus_new_tcp(3)

       TCP PI (IPv4 and IPv6) Context
	   The TCP PI (Protocol Indepedent) 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 context
	       modbus_new_tcp_pi(3)

       Common
	   Before using any libmodbus functions, the caller must allocate and
	   initialize a modbus_t context with functions explained above, then
	   the following functions are provided to modify and free a context:

	   Free libmodbus context
	       modbus_free(3)

	   Set slave ID
	       modbus_set_slave(3)

	   Enable debug mode
	       modbus_set_debug(3)

	   Timeout settings
	       modbus_get_byte_timeout(3)modbus_set_byte_timeout(3)modbus_get_response_timeout(3)modbus_set_response_timeout(3)

	   Error recovery mode
	       modbus_set_error_recovery(3)

	   Setter/getter of internal socket
	       modbus_set_socket(3)modbus_get_socket(3)

	   Information about header
	       modbus_get_header_length(3)

	   A libmodbus context is thread safe and may be shared among as many
	   application threads as necessary, without any additional locking
	   required on the part of the caller.

	   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_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]

	   Handling of bits and bytes
	       modbus_set_bits_from_byte(3)modbus_set_bits_from_bytes(3)modbus_get_byte_from_bits(3)

	   Set or get float numbers
	       modbus_get_float(3)modbus_set_float(3)modbus_get_float_dcba(3)modbus_set_float_dcba(3)

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

       Establish a connection
	   modbus_connect(3)

       Close a connection
	   modbus_close(3)

       Flush a connection
	   modbus_flush(3)

   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:

       Read data
	   modbus_read_bits(3)modbus_read_input_bits(3)modbus_read_registers(3)modbus_read_input_registers(3)modbus_report_slave_id(3)

       Write data
	   modbus_write_bit(3)modbus_write_register(3)modbus_write_bits(3)modbus_write_registers(3)

       Write and read data
	   modbus_write_and_read_registers(3)

       Raw requests
	   modbus_send_raw_request(3)modbus_receive_confirmation(3)

       Reply an exception
	   modbus_reply_exception(3)

   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: modbus_mapping_new(3) modbus_mapping_free(3)

       Receive
	   modbus_receive(3)

       Reply
	   modbus_reply(3)modbus_reply_exception(3)

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(3).

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.

AUTHORS
       The libmodbus documentation was written by Stéphane Raimbault
       <stephane.raimbault@gmail.com[1]>

RESOURCES
       Main web site: http://www.libmodbus.org/

       Report bugs on the issue tracker at
       http://github.com/stephane/libmodbus/issues.

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 files COPYING
       and COPYING.LESSER included with the libmodbus distribution.

NOTES
	1. stephane.raimbault@gmail.com
	   mailto:stephane.raimbault@gmail.com

libmodbus 3.1.0			  10/06/2013			  LIBMODBUS(7)
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