C++ Embedded Modbus Library
Features:
- object oriented C++
- currently supports Modbus RTU slave
- most methods and functions are unit tested
- easy to use - in most cases just call the correct poll() method in the main loop
- liberal license (MIT)
- non-blocking state machine based RTU framer design
------------- -------------
+--Sent--->| TX CRC | +--->| TX Wait |
| ------------- | -------------
| | | |
------------- Sent TX Empty T3.5 start
| TX PDU | | | | |
------------- v | | v
^ ------------- | v -------------
| | TX Drain |----+ +------+<----T3.5-----| Dump |
Sent ------------- | -------------
| v ^
------------- ------------- |
| TX Addr | +--begin_send()--| Idle |----Invalid Char0--->+
------------- | ------------- ^
^ | ^ | |
| +---+ | Addr. Match |
send() | +----------+ | |
| v | v |
------------- fini- | T3.5+ ------------- |
| Queue |-shed()->+<--CRC/--| Receive |---T1.5/Comm Err-->+
------------- ^ F.E. ------------- ^
| ^ | | |
| | | T3.5+Frame OK finished()/send()
| begin_send() finished() | |
| | | v |
| | | ------------- -------------
| +-------------+---------| Frame Ready |--Receive-->| Collision |
| ------------- -------------
Receive ^
| |
+----------------------------------------------------------------+
For class descriptions see the src/ModbusInterface.h file.
This project follows the Arduino library format version 2, so you can select "Download ZIP" from the github page and then select the "Add Library..." option from the Arduino environment to easily import this library.
The "modpoll" command line utility is highly recommended for accessing slaves. It can be found here: http://www.modbusdriver.com/modpoll.html
To install it, copy the executable to some location in your path, such as C:\Windows (for Windows) or /usr/local/bin (for Linux).
You can read the a holding register (4xxxxx) with modpoll using the
following command line:
modpoll -m rtu -a <slave address> -t 4 -r <register> -b <baud> <COMx:>
You can write to a register using the following:
modpoll -m rtu -a <slave address> -t 4 -r <register> -b <baud> <COMx:> <value>
Replace <slave address> with the address of your slave (i.e. 1), <register>
with the 1 based register number (i.e. 1=40001), <baud> with the baud rate
(i.e. 19200), <COMx:> with your COM port (i.e. COM4: or /dev/ttyS1) and
<value> with the value to write. You may omit -m rtu on if the device
name starts with COM, and you may omit -b <baud> if the baud rate is
19200.
For example, to read the first holding register (40001) from slave id #1 on
COM3: at 19200 baud, you can use the following:
modpoll -1 -a 1 -t 4 -r 1 COM3:
To write the value 100 to the above register, use the following:
modpoll -a 1 -t 4 -r 1 COM3: 100
Please be aware that modpoll will assert the DTR line when it opens the port,
which will usually cause an Arduino to reset. You may need to disable the DTR
reset support on the board, or use a serial to TCP bridge program like piracom
(http://www.pira.cz/show.asp?art=piracom) which keeps the port open and connect
to the TCP socket instead. If you use a serial to tcp bridge, change the
-m rtu command line switch to -m enc.
To use modpoll to read the above register through piracom running on the local
machine, use:
modpoll -1 -m enc -a 1 -t 4 -r 1 localhost