pci-6229 wrong samples

[tudmuis]

Hi all,

I have two PCI-6229 boards which i use under RT-Suse Linux Enterprise and comedilib. When i program the card for multiple acquisition i get a wrong amount of samples returned.

I tried to change the amount of channels for 1 scan and i get an extra sample when i get above 26 channels. When i change the amount of scans to two i got the correct amount of samples but when i lower this amount too 16 channels and two scans i get five extra samples. Does anyone has an idea?

tudmuis

[ninlmartijn]

Dear tudmuis,

thank you for your post on our webforum. I'm not familiar with comedilib, so it is hard so support you with this. I didn't see this behavior in our software environment (under Windows or Real Time). With these cards the idea is to create a task; with the task you tell the DAQ board what to do. For example:

- create a channel for physical channel 0-18 (so 19 channels)

- the type of task will be finite; N samples at rate 1000 s/s, the minimum samples per channel is with this type of task 2 so 38 samples will be taken

- then you start the task

- we can read N samples on N channels or we can read twice 1 sample on N channels

I hope this gives you a little bit insight. If you still are getting the wrong number of samples, it is good to determine if you are measuring extra channels or if you are reading some channels twice. Are you measuring extra samples if you are trying to sample on all available channels?

Again it is hard to support you with this sine we don't see this behaviour with our driver and environment. The Comedi website also states that 'Comedi and Comedilib are being actively developed, and because of this, new versions are sometimes buggy.'     

Best regards,

[tudmuis]

Dear Martijn,

i create a scanlist with 16 channels differential mode with a conversion time of 7000 nanoseconds and a minimum sample of 2. This will create 64 bytes of information in the buffer.

The fastest frequency will be then 7812.5 Hz. The total time i spend on the scan and read of the buffer takes me 466 microseconds.

This results in a frequency of my program close too 2000 Hz. Can you explain me why it takes so much time to readout the buffer? And why i have to read minimum of two samples/channel?

If the time can't be changed does NI has a possibility to exchange cards with the PCI-6529? (the two cards are 1 month old)

Andries