daqmx dotnet multiple devices conversion time error

[DaveG-GJ]

I have two PXI systems, each contining multiple modules.  System 1, the newest, is a PXI-1031DC chassis containing two PXI-6123 modules (8 channel, 500kHz, S-series ADC).  System 2, the older, is a PXI-1042 containing seven PXI-6120 modules (4 channel, 800KHz, S-series ADC).

I am using DAQmx V8.1 with Visual Studio 2005.

I am using a single analog task to read all the (simultaneously sampled) A/D channels.  The code works fine on System 1 (with 16 channels) but it does not work fine on system 2 (with 28 channels).  It consistently throws the error "ADC conversion attempted before prior conversion complete." 

The error does not occur if a single module (4 channels) is selected but it does occur if two or more modules are selected (8 or more channels).

The error can be made to go away by slowing the sampling rate to something on the order of 10% of the module's capability.  The error is intermittent when sampling rate is near some threshold.  The error also seems to be dependent on the number of samples specified (using an N-sample block).

The error occurs identically when a task is created and tested within MAX.  So this seems tends to rule out errors in my code.  Additionally, the fact that the same code runs fine on the newer system tends to rule out errors in my code. 

The system runs fine using old code that samples 28 channels at 800 KHz/channel, in blocks of 80000 points per channel.  This 'old' code is VB6 and traditional NIDaq.  The ability to run this code rules out problems with the hardware.

If anyone else has seen this problem, I would be appreciative of any information.  The problem certainly seems to be a problem with DAQmx and the 'older' PXI6120 modules.

[Adam W]

Hi Dave,

There is a knowledge base that has some general information on that error that might be helpful to look at:

http://digital.ni.com/public.nsf/websearch/A01AA72779C4604986257047007344EC?OpenDocument

Hope this helps!

Adam W
National Instruments

[DaveG-GJ]

Adam,

Thanks for trying.  The article doesn't have much that is applicable.  I am not using an external trigger and the trigger rate is explicitly set at the beginning in the task.  The problem can easily be reproduced by setting up a task in MAX.

I am solving the problem temporarily or maybe permanently by using separate tasks for each module.  My problem is, I believe, a bug or an inefficieincy in DAQmx 8.1. 

Dave