How I can use the datalogger:
In the case from the data logger a switch is inserted to connect the accumulators with the Gnublin.
After switching on, Linux launsch, the programms dmm and my_shutdown.
Dmm reads the data of the digital multimeter, and stores the data in the file "data.txt ". The programm "My_shutdown" measures the voltage of the accumulators.
A LED is inserted also in the case lid. A full accumulator flashes the LED once and makes afterwards a break. The lamp flashes accordingly the accumulator filling.
If the accumulator is empty, the LED flashes 5 times per period.
Moreover, a button is inserted.
After the measurement is concluded, the button has to be pressed. The operating system linux is driven down.
15 seconds, after the last Led flash, the data logger can be switched off.
Now the dates can be read by the SD card with a personal computer.

How can I build the datalogger:
There is a live CD ""; on the web page for development.
Content of the CD is a great developing surrounding for c/c++ with Eclipse.
The SD-card for the gnublin can be also generated with this CD.
1. Install the linux system on SD-card for the gnublin.
2. Load the program files from Internet. (http://www. atwillys. de/content/cc-apps/vc920-dmm-acquisition-linux-macos/)
3. At the time this original programm is not able to store the received data.
Therefore, it still must be extended. "new_main.txt" describes changes which must be still inserted in the file "main.c".
4. Now you can compile the programm with "make" (compile on gnublin). Make needs his "Makefile" and the following Files: main.c, serialport.c, vc920.c, serialport.h, vc920.h.
The generated progamm name is "dmm". I stored it in the path "/measurment/dmm-vc920/"
5. The a program to measure the accu and shut down the gnublin also must be compiled.
Therefore the files main.cpp, gnublin.h and gnublin.cpp must be compiled on gnublin.
The command is "g++ main.cpp gnublin.cpp -o my_shutdown -lpthread"
The file "my_shutdown" must be copied to "/measurment".
6. The data logger has no keyboard and no monitor. Therefore, some program parts must be loaded while starting. This automatically happens with a bash file (see file "").
First the IO ports are adjusted. Then the drivers are loaded (USBtoSerial and AnalogtoDigital). Afterwards starts the data logger program and the battery supervision program.
This is done by the file "" in the folder "/measurment"
7. Therefore we must modify the file "rc.local". The text "/measurment/" must be inserted in the previous line "exit 0".
The file rc.local is a part of the folder "/etc".
8. The datalogger should store the data in the file "data.txt", which is included in the folder "/measurment/dmm-vc920/". The access rights from "data.txt" must be changed with the order "chmod 666 data.txt". Now the SD-Card can be read with a PC.

If you have problems, you can also google for "raspberry pi". So I found all answers for my questions.

If you use Windows, you need a programm to read the Linux file-system (ext4).
Gnublin supports FTDI- und CP210x-drivers, use such usb2serial devices.