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Command line tool to access Modbus devices

Project description

Date: 2021-11-23
Copyright: MPL 2.0
Version: 0.1.7
Manual section:1


modbus [-h] [-r REGISTERS] [-s SLAVE_ID] [-b BAUD] [-p STOP_BITS] [-P {e,o,n}] [-v] device access [access …]


Read and write registers of Modbus devices.

Access both TCP and RTU (i.e. serial) devices and encode and decode types larger than 16 bits (e.g. floats) into Modbus 16 bits registers.

Optionally access registers by symbolic names, as defined in a registers file. Symbolic names for enumerations and bitfields are supported too.

Designed to work nicely with other standard UNIX tools (watch, socat, etc.), see the examples.

Implemented in python on top of the protocol implementation provided by the umodbus python library.


Regular python install, either pip install modbus_cli to install from pypi or python install to install from source.


/dev/ttyXXX for serial devices, or hostname[:port] for TCP devices
One or more read or write operations. See ACCESS SYNTAX below.
-r FILE, --registers=FILE
 Read registers definitions from FILE. Can be specified multiple times.
-v, --verbose Print on screen the bytes transferred on the wire.
-b BAUD, --baud=BAUD
 Set the baud rate for serial connections.
-p BITS, --stop-bits=BITS
 Set the number of stop bits for serial connections.
-P PARITY, --parity=PARITY
 Set the parity for serial connections: (e)ven, (o)dd or (n)one
-B ORDER, --byte-order=ORDER
 Set the byte order to one of ‘le’ (little endian), ‘be’ (big endian) or ‘mixed’
-h, --help Show this help message and exit.



Mnemonic: access the register(s) of MODBUS_TYPE starting at ADDRESS, interpreting them as BINARY_TYPE. The / syntax is inspired by gdb (but the available types are different.)

MODBUS_TYPE = h|i|c|d

The modbus type, one of

code name size writable
h holding register 16 bits yes
i input register 16 bits no
c coil 1 bit yes
d discrete input 1 bit no

The default modbus type is holding register.

ADDRESS = <number>
0-based register address
BINARY_TYPE = <pack format>

Any format description accepted by the python standard pack module. Some common formats are:

code type
h 16 bits signed integer
H 16 bits unsigned integer
i 32 bits signed integer
I 32 bits unsigned integer
f 32 bits IEEE 754 float

The default byte order is big-endian. To use little endian, use a < prefix, or --byte-order=le. To use mixed endian (little endian, but individual 16 bit registers are big endian), use --byte-order=mixed.

VALUE = <number>

The value to be written to the register. If not present, the register will be read instead.

If only one register is written to, the modbus function 6 (0x6), “write single register” is used. If multiple registers are written to, the modbus function 16 (0x10), “write multiple registers” is used.


Read a holding register

$ modbus $IP_OF_MODBUS_DEVICE 100

Write a holding register

$ modbus $IP_OF_MODBUS_DEVICE 100=42

Read multiple registers

To read (or write) multiple registers simply list them on the command line:

$ modbus $IP_OF_MODBUS_DEVICE 100 c@2000

When performing access to multiple contiguous registers, one single modbus operation is performed.

When multiple modbus operations are needed, they are all initiated at once, and the results are collected as they arrive.

More examples of the access syntax

h@39/I read the 32-bits unsigned integer stored in holding registers at addresses 39 and 40
39/I same as above (h is the default modbus type)
39/I=42 write the integer 42 to that register
SOME_REGISTER=42 same as above, provided the registers file contains the definition SOME_REGISTER h@39/I
39/I=0xcafe the value can be specified in hexadecimal
c@5 read coil at address 5
h@24/f=6.78 write a floating point value to holding registers at addresses 24 and 25
i@1/6B read six unsigned bytes stored in input registers at addresses 1, 2 and 3

Monitor a register

The UNIX command watch can be used to read a register at regular intervals:

$ watch modbus $IP_OF_MODBUS_DEVICE 100

Read a serial device attached to a remote computer

The UNIX command socat can be used to access a remote device through a TCP tunnel:

remote$ socat -d -d tcp-l:54321,reuseaddr file:/dev/ttyUSB0,raw,b19200
local$ socat -d -d tcp:sc:54321 pty,waitslave,link=/tmp/local_device,unlink-close=0
local$ modbus /tmp/local_device 100

Read multiple registers based on their names

Given the following registers definitions:

$ cat registers.modbus
di0 d@0
di1 d@1
ai0 i@512
ai1 i@513

glob matching (*, ?, etc.) can be used to read all the ai registers at once:

$ modbus -r registers.modbus $IP_OF_MODBUS_DEVICE ai\*


The purpose of the registers files is to be able to refer to registers by name.

There can be multiple definition files, specified using either the -r command line switch or the MODBUS_DEFINITIONS environment variable.

A # in a definition file starts a comment.

Each line contains a symbolic name followed by a register definition. The name and the definitions are separated by spaces, for example:

status i@512:STATUS
leds 513:LEDS

The file can also contain the possible values for an enumeration or a bitmask, for example:

# This is an enumeration named STATUS

# This is a bitmask named LEDS


A colon separated list of register definitions files.

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