Sometimes when you are dealing with automation products, the modbus protocol is a very interesting alternative due to its simplicity and efficiency. At my work for Apel, we are currently developing some interesting applications for metro stations using GPS Antennas and Embedded Linux Boards like the TS-7400 from Technologic Systems, the linux distribution chosen was mainly debian due to the fact that it comes always almost with everything we need and Idid not want to have many additional work on that aspect(do not tell my boss on that one 
),as we are planning to work some aspects of the product further.
Moreover, I would like to share just a bit of the experience as it may help more people to get their projects more advanced. For that I have configured a simple situation where I had a GPS antenna, a board and an old computer running an almost as old Windows XP. Then, I have connected both of them using the serial port and a serial/TTL board converter that we have constructed.
ntpGPS Modbus
Then, for programming I have used python and a nice project called Modbus-Tk http://code.google.com/p/modbus-tk/, an interesting program for running the master to test is the Modbus Tester that you can download from http://modbus.pl/Modbus_Tester.html, a different approach(and I did that already, it is indeed very simple) is to construct your RTU master using the modbus-tk library, as it comes with simple to use methods.
Using this program you can test communication with modbus slaves. At present, only the Modbus RTU mode is available.
The modbus content consisted mainly in:
Modbus Request
Modbus feedback
For analyzing the information, you need to make some conversion such as the one, below:
converting Hexadecimal to Decimal
At the code side you should mostly , due something like
slave_1 = modbusServ.add_slave(1)
slave_1.add_block(“a”, cst.ANALOG_INPUTS, 200,7)
and at the program you should set the values with the system time, like:
slave_1.set_values(“a”, 200, hour)
…
Video:
