Python serial data aquisition

[Flavio codeco coelho]

bokr at oz.net (Bengt Richter) wrote in message news:<41e384d0.552090624 at news.oz.net>...
> On 9 Jan 2005 14:13:28 -0800, fccoelho at gmail.com (Flavio codeco coelho) wrote:
> 
> >Hi,
> >
> >I am using pyserial to acquire data from an A/D converter plugged to
> >my serial port.
> >
> >my hardware represents analog voltages as 12bit numbers. So, according
> >to the manufacturer, this number will be stored in two bytes like
> >this;
> > |-------------bits(1-8)-----------|
> >Byte1: x x x n1 n2 n3 n4 n5
> >Byte2: x n6 n7 n8 n9 n10 n11 n12
> >
> >where x is some other information, and nx are the digits of my number.
> >
> >My problem is to how to recover my reading from these bytes, since
> >pyserial gives me a character (string) from each byte... I dont know
> >how to throw away the unneeded bits and concatenate the remaining
> >bits to form a number...
> >
> The others have shown how to recover a 12 bit positive value 0 through 4095,
> but if the number is signed, and you want the signed value, you'll have to
> find out how signed numbers are represented. An offset is common, in which
> case you would subtract 2048 (2**11). If it's two's complement by some chance,
> you'll want (I think -- untested ;-) to do num -= 2*(num&2048) instead of always num -= 2048
> If you need speed in converting large strings of byte pairs, you could
> convert pairs of bytes with the array module ('H' for unsigned 2-byte numbers)
> and use the resulting 16-bit numbers as indices into another array of final values
> prepared beforehand with redundant information that will accomplish the effect
> of masking and shifting and adjusting sign.
> If you need this speed, volunteers will magically appear. Maybe even if you don't ;-)
> Regards,
> Bengt Richter


Hi Bengt,

The Idea of using Array is realy cool Though I have to think about it
since I would to plot the values as they are sampled...

Anyway, how would you set up this array to do the shifting and
masking?

BTW, since this thread is generating quite a bit of attention let me
post a complete description of my problem so that it may serve as
reference to others:

Hardware: DI-151RS from Dataq (2 analog channels, 2 digital input,
single ended/differential recording, max sampling rate 240Hz) connects
to the serial pro through a standard db9 plug.

Encryption table:


       B7      B6      B5      B4      B3      B2      B1      B0
Byte1   A4      A3      A2      A1      A0      1       Din     0
Byte2   A11     A10     A9      A8      A7      A6      A5      1
Byte3   B4      B3      B2      B1      B0      1       Din     1
Byte4   B11     B10     B9      B8      B7      B6      B5      1

first two bytes are for analog ch 1 and remaining two are for ch 2.
Din stands for digital in. AXX and BXX are the nth bits of each
reading. A0 and B0 are the least significant bits.

The latest and preferred solution on how to convert these bytes is,
according to the suggestion of Chris Liechti (author of pyserial) is:
(this is for the first channel only, repeat for the second)

import struct

 l, h = struct.unpack(">BB", ser.read(2))
 n = (l >> 3) + ((h >> 1)<<5)

struct.unpack returns a tuple of values represented by a string(the
output of the read command) packed according to the format specified
by ">BB"
In this forma string, ">" stands for big Endian representation and "B"
stands for unsigned char.

If anyone has a better suggestion, speack up!

oof! I started this thread knowing next to nothing about this stuff,
It seem that I am finally getting the idea! :))

cheers,

Flávio