Question

I m new to the world of Python and my programming skills are fairly poor but I m trying to figure a way to use Python to display the output from an EEG circuit (using the OpenEEG circuit http://openeeg.sourceforge.net)

The analogue output is amplified and processed via an ADC (in an ATmega8 microcontroller) and is converted to RS232 by a MAX232.

The RS232 Signal is as follows:

Byte 1: sync value 0xa5
Byte 2: sync value 0x5a
Byte 3: version
Byte 4: Frame number
Byte 5: Channel 1 Low Byte
Byte 6: Channel 1 High Byte
Byte 7: Channel 2 Low Byte
Byte 8: Channel 2 High Byte
...
Bytes 9-16 are for extra electrode channels but data not required since only using the first two
...
Byte 17: Button states (b1-b4)

I ve got some basic PySerial functionality but I need to figure a way to make use of the incoming data by buffering it and plotting the useful values as 2 real-time x-y waveforms (time vs voltage)

Question update:

I m getting the code to print with the obvious few lines of PySerial but it s gibberish. I m trying to strip the data down in to the format of values that can then be plotted. The 17 bytes of data is currently coming in at 256 frames/sec. The (two) channels are made up 10 bits of data each (with 6 zeros to make up the rest of the 2 bytes). They are unsigned giving possible values of 0 to 1023. These correspond to values that should be plotted as positive and negative, so a binary value of 512 corresponds to a plot of zero (micro)volts....

How do I read the incoming stream as 8 bit binary (stripping out the data that I don t need), then combine the two relevant bytes from each channel that I want (possibly removing the surplus 6 zeros if necessary)?

Answer 1

To handle the complicated binary data format you could maybe use structured arrays in numpy (see also here for a nice introduction). After defining the structure of the data it should be very easy to read it in. Then you could use numpy s functionality to cook down the data to what you need.

Answer 2

There s a good realtime plotting example here. It s a good example in that it runs with self generated data, so it s easy to test, but it s also obvious where to modify the code for plotting real data, and the code is easy to follow.

The basic idea is to make a plot window and then update that data as it comes in using

set_xdata(np.arange(len(self.data)))
set_ydata(np.array(self.data))

(Though in the current versions of matplotlib you may want to use set_data(xdata, ydata) instead.)

As for parsing the serial port data, it s probably better to ask that as a separate question.

友情链接