Command line tool to access Modbus devices
- MPL 2.0
- Manual section:
modbus [-h] [-r REGISTERS] [-s SLAVE_ID] [-b BAUD] [-p STOP_BITS] [-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.
Implemented in python on top of the protocol implementation provided by the umodbus python library.
/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.
- -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
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:
16 bits signed integer
16 bits unsigned integer
32 bits signed integer
32 bits unsigned integer
32 bits IEEE 754 float
The default byte order is big-endian, use a < prefix in the format to specify little-endian.
- VALUE = <number>
The value to be written to the register. If not present, the register will be read instead.
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
More examples of the access syntax
read the 32-bits unsigned integer stored in holding registers at addresses 39 and 40
same as above (h is the default modbus type)
write the integer 42 to that register
same as above, provided the registers file contains the definition SOME_REGISTER h@39/I
the value can be specified in hexadecimal
read coil at address 5
write a floating point value to holding registers at addresses 24 and 25
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 :STATUS 0=OFF 1=ON 2=ERROR # This is a bitmask named LEDS |LEDS 0=LED0 1=LED1 3=LED3 4=LED4
A colon separated list of register definitions files.
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