/
libmodbus.txt
184 lines (137 loc) · 5.76 KB
/
libmodbus.txt
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
libmodbus(7)
============
NAME
----
libmodbus - 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 etablish 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).
Many Modbus devices can be connected together on the same physical link so you
need to define which slave is concerned by the message with
linkmb:modbus_set_slave[3]. If you're running a slave (server) the slave number
is used to filter messages.
Create a Modbus RTU context::
linkmb:modbus_new_rtu[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::
linkmb: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::
linkmb: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::
linkmb:modbus_free[3]
Context setters and getters::
linkmb:modbus_set_debug[3]
linkmb:modbus_set_error_recovery[3]
linkmb:modbus_set_slave[3]
linkmb:modbus_get_timeout_begin[3]
linkmb:modbus_set_timeout_begin[3]
linkmb:modbus_get_timeout_end[3]
linkmb:modbus_set_timeout_end[3]
linkmb: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.
Connection
~~~~~~~~~~
The following functions are provided to establish and close a connection with
Modbus devices:
Establish a connection::
linkmb:modbus_connect[3]
Close a connection::
linkmb:modbus_close[3]
Flush a connection::
linkmb:modbus_flush[3]
Data
~~~~
The Modbus protocol defines different data types and functions to read and write
them from/to remote devices.
Read data::
linkmb:modbus_read_bits[3]
linkmb:modbus_read_input_bits[3]
linkmb:modbus_read_registers[3]
linkmb:modbus_read_input_registers[3]
Write data::
linkmb:modbus_write_bit[3]
linkmb:modbus_write_register[3]
linkmb:modbus_write_bits[3]
linkmb:modbus_write_registers[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
linkmb: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>
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). For details see the files `COPYING` and `COPYING.LESSER`
included with the libmodbus distribution.