Re: PCI-6251 strange input channels reading

On Wednesday 25 April 2007 14:18, sergey tereshko wrote:
> I used the comedi_data_read_n(....,10) for all channels and the output
> looks like this:
>
> Device opened.
> channel 0: 0.00 0.00 -0.38 -0.38 -0.38 -0.38 -0.38 -0.38 -0.38 -0.38
> channel 1: 0.00 0.00 -2.50 -2.50 -2.50 -2.50 -2.50 -2.50 -2.50 -2.50
> channel 2: 0.00 0.00 -2.55 -2.55 -2.55 -2.55 -2.55 -2.55 -2.55 -2.55
> channel 3: 0.00 0.00 -2.52 -2.52 -2.52 -2.52 -2.52 -2.52 -2.52 -2.52
> channel 4: 0.00 0.00 -2.56 -2.56 -2.56 -2.56 -2.56 -2.56 -2.56 -2.56
> channel 5: 0.00 0.00 -2.51 -2.51 -2.51 -2.51 -2.51 -2.51 -2.51 -2.51
> channel 6: 0.00 0.00 -2.55 -2.55 -2.55 -2.55 -2.55 -2.55 -2.55 -2.55
> channel 7: 0.00 0.00 0.00 -2.47 -2.47 -2.47 -2.47 -2.47 -2.47 -2.47
> channel 8: 0.00 0.00 0.98 0.98 0.98 0.98 0.98 0.98 0.98 0.98
> channel 9: 0.00 0.00 1.22 1.22 1.22 1.22 1.22 1.22 1.22 1.22
> channel 10: 0.00 0.00 1.41 1.41 1.41 1.41 1.41 1.41 1.41 1.41
> channel 11: 0.00 0.00 1.68 1.68 1.68 1.68 1.68 1.68 1.68 1.68
> channel 12: 0.00 0.00 1.88 1.88 1.88 1.88 1.88 1.88 1.88 1.88
> channel 13: 0.00 0.00 2.16 2.16 2.16 2.16 2.16 2.16 2.16 2.16
> channel 14: 0.00 0.00 2.41 2.41 2.41 2.41 2.41 2.41 2.41 2.41
> channel 15: 0.00 0.00 2.78 2.78 2.78 2.78 2.78 2.78 2.78 2.78
> Done.
>
> 1 channel was hooked up to -2.5V, the other ones are in the air.
> what is wrong about these first two readings(on rare occasions the first
> three readings are zeros)? and why did you advise to use more than 4
> readings?
>
> Thanks, Frank for your help.

Because there is some fifo flushing code for the 625x boards that I added 
after seeing it done in the NI example code.  It does 3 bogus conversions.  
I obviously didn't think much about what it was really supposed to do when 
I added it though.  I think it should be doing the 3 bogus conversions 
after the real one, to move the real sample so it becomes available in the 
board's fifo.  Instead, it's doing the 3 bogus conversions after clearing 
the fifo and before setting up the board for the real sample.

-- 
Frank