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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
o MODBUS_GET_HIGH_BYTE(data), extracts the high byte from a
byte
o MODBUS_GET_LOW_BYTE(data), extracts the low byte from a
byte
o MODBUS_GET_INT32_FROM_INT16(tab_int16, index), builds an
int32 from the two first int16 starting at tab_int16[index]
o MODBUS_GET_INT16_FROM_INT8(tab_int8, index), builds an
int16 from the two first int8 starting at tab_int8[index]
o 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 Stephane 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)