Interfacing a Modbus/TCP system
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original in en Guido Socher
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Guido likes Linux because it is a really good system to
develop your own hardware.
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Modbus is an old protocol introduced in 1979 by the
PLC manufacturer Modicon. A PLC is a kind of first generation microcontroller.
The modbus protocol became a success because it was
easy to use and could be applied to many applications.
It continues to be standard in industrial controllers and is still used
in industrial and home automation systems.
More information about Modbus protocol can be found at:
Modbus is essentially a master slave protocol that
can be used to read and write registers in some device.
Initially it was made for systems that run on RS232 communication.
Today many devices talk Modbus/TCP which is the Modbus protocol
embedded into a TCP/IP connection.
We use the tuxgrahics avr ethernet board as a TCP/IP client
talking to a Modbus/TCP server.
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How Modbus/TCP works
Modbus/TCP places a special version of the Modbus protocol into
a tcp/ip communication. Port 502 is used on the server side for
this purpose. The Modbus protocol consists of a
header part and a data part. The fields in the header part
The actual data part of a query consists then of commands to
read/write a given single bit or a number of 16 bit registers.
- 2 byte transaction identifier
- 2 byte protocol identifier (always zero, 0=Modbus)
- 2 byte length, number of bytes following
- 1 byte unit identifier, ID of the remote slave connected on a serial line o on
other buses reachable via the Modbus/TCP server.
The project presented here is a TCP/IP client. In other words
we implement a Modbus/TCP client that talks to a Modbus/TCP server.
The Modbus/TCP server has the actual devices attached where we
want to read/write data.
A Modbus/TCP request decoded in wireshark
The example: A web to Modbus/TCP gateway
There is no general purpose application for such a Modbus/TCP client.
It is useful if you have some automation equipment that appears as
a Modbus/TCP server and you want to interface to it. Maybe you want
to build your own web-interface to it or you have some other
application in mind that you want to run on the tuxgraphics ethernet
board and it should talk to this automation equipment.
In any case the code in the download section is an example and you
can integrate it into your own ideas. Sending a query out of the
tuxgraphics ethernet board works in 3 simple steps:
- You call function client_tcp_req like this:
This starts a TCP transaction.
- The function modbus_datafill_callback will then be called
when it is time to fill in the modbus/tcp query data. This happens
normally a few milliseconds after you called client_tcp_req.
- The function modbusresult_callback will be called when
the answer from the server is back and you get the result
passed to this function.
The callback functions are a good way to integrate any user code
into the state-machine of a communication protocol. They save
storage memory and allow a direct interaction with the protocol flow.
Think of them like interrupt routines which can be triggered after
you called client_tcp_req.
Open file main.c to see how this works.
This example application from the download section is a web to
modbus/tcp gateway. It allows you to enter an arbitrary hex
string which will be sent out as modbus/tcp query and it displays
the answer (in hex).