Strangeness with cb_pcidas and 1602/16 board

I am experiencing slight interference between two channels on the
Computerboards PCI-DAS1602/16.  I am not sure if this is due to the driver
or due to some quirk of the board, but here is how I can best describe the
scenario:

Basically I have some code that loops through a set of channels (say
channel 1 and channel 2) and issues comedi_data_read()s for each of the
channels in the set.  The code then sleeps until the next period.  (Each
of these loop iterations is a scan and the scan rate is 1kHz.)

At any rate, I noticed completely odd interference behavior.  Basically, I
have a function generator generating a triangular wave with a period of
like 3Hz connected to AI channel 1, and a heartbeat signal simulator
connected to AI channel 2.  If I turn channel 1 on and channel 2 on in
software (meaning I flag them as candidates for comedi_data_read()  in my
periodic loop), I noticed that the waveform in channel 2 is interfered
with slightly by the waveform in channel 1.

However, if I disable channel 1 in software, the interference pattern
disappears.

***Even though channel 1 is still physically connected to its signal
source**.

Basically, the simple act of issuing a comedi_data_read() on channel 1
affects/modifies the waveform I get out of channel 2.

Is this odd or what?

At first I thought that maybe this was because my two comedi_data_read()s
were issued one after another, and I was hitting some limit on how fast
you can talk to this board.  However, introducing some usleeps in the two
successive comedi_data_read()s for the two channels doesn't help matters,
so it isn't the fact that the comedi_data_read()s were happening one after
the other....

Has anyone else experienced strange interactions between channels with
this board?

Normally I would suspect my cabling.. but since the only difference
between getting a clean signal and a noisy signal is whether or not I
issue comedi_data_read()s on different combinations of channels, I decided
to ask if it could *possibly* be a problem in software rather than
hardware/cabling.

Any ideas?


Thanks,


-Calin

Received on 2002-08-08Z21:44:23