External Clock PCI-6110

[remifab]

Hello / Bonjour

I am using Labview 6.1 / Window 2000 and a NI PCI-6110 card.
The system that I develop uses an external clock (connected to the PFI7) to sample a signal in channel 0, a trigger connected to TRIG 1 starts an acquisition. The program controlling the DAQ is a modified version of “Acquire N Scans ExtScanClk D-Trig.vi” example, I have removed from this example the two sub-vi “AI Clock Config”. The program uses then the STARTSCAN signal as an external clock.

I have three questions:

1 How do you configure STARTSCAN to either the rising or falling edge polarity as it is stipulated in the pci-6110 user manual?

2 What is the purpose of the two sub-vi “AI Clock Config” in the “Acquire
N Scans ExtScanClk D-Trig.vi” as it seems the PCI-6110 card doesn’t need them to run properly with an external clock?

3 Should I configure the CONVERT* signal if I would like to sample the channel 0 signal immediately when a STARTSCAN pulse appears?

Thank you

[rgharrison]

Hi,

I'll try to answer your questions.
1) In the AI Clock Config.vi you can select the cluster "clock source". In here you will have the option to select a PFI pin and select either low to high or high to low. This will determine the polarity of your SCAN CLOCK.

2) The purpose of the two subVIs "AI Clock Config.vi" are to first, read the maximum channel clock rate you can have and then second, change the channel clock so that it is the fastest rate possible. The channel clock is responsible for sampling the each channel during a given scan clock period. Since we are going to use an external scan clock, it is important to make the channel clock rate as fast as possible since it is possible that the external scan clock is faster than the channel clo
ck. If your scan clock rate is slow enough, then removing these two VIs will not change how the program works (as you mentioned).

3) The CONVERT* signal is only used if you want to clock your A/D conversion externally (or read the CONVERT*). This is essentially the physical representation of the channel clock. You shouldn't need to clock this signal externally to have your signal immediately perform an A/D conversion when you STARTSCAN signal appears. It should happen automatically.

Anyway, hope that helps. Have a good day.

Ron

[remifab]

Thank you rgharrison for your answer

I tried to make some modifications and tests following your comments.
I have incorporated the �AI Clock Config� in my application (which is now very similar to �Acquire N Scans ExtScanClk D-trig.vi� except that there is only one �AI Clock Config� VI) and I have these questions and comments:

As the PCI-6110 samples each channel simultaneously (and I want to sample only one channel), should I set the channel clock as you mentioned?

Given that my external clock is connected to PFI7 and I have specified �PFI pin, high to low� in �clock source code�, it doesn�t matter for the board if I set any PFI as �clock source string�: my board still find an external clock. But if I leave �clock source string� blank I�ve got
an error. I noticed also that the parameters of the �actual clock rate specification� cluster are still equal to zero. Is it normal or I misunderstand something?

I don�t understand why I should set up the �clock source� as fastest as possible, as I want to use an external clock and sampling my signal one time per external clock period. Also, in the �Acquire N Scans ExtScanClk D-trig.vi� exemple, after set the clock as fast as possible, the following VI �Ai Start� disable the internal clock, so again why set up the clock and disable it just after?

If I set the �clock source� at the maximum rate and if the scan clock (my actual external clock) is slower than the �clock source�, can the board acquire more than one sample by external clock period?

Regards

[rgharrison]

Hi,

If you are simply looking for analog input using an external clock for 1 channel, the example you were using is perfect as is (Acquire N Scans ExtScanClk D-Trig.vi). Your scan clock should be connected to PFI7.

There are two clocks in this data acquisition system, a scan clock and a channel clock. The scan clock indicates when all your channels will be sampled. The channel clock indicates when each channel is sampled when sampling multiple channels on a multiplexing board. Since the PCI-6610 is a simultaneously sampling board, there is only 1 clock, the scan clock. This is because each channel is sampled simultaneously and not multiplexed therefore there is no need for a multiplexing clock (the channel clock). Therefore, you are
correct, you don't need that extra clock configuration code before the AI Start.vi.

To answer your question as to why you would want the channel clock (multiplexer clock) to be setup as fast as possible for an external clock, it is because since the board doesn't know the exact rate you are sampling at, it is important to multiplex each channel as fast as possible just in case you are using a fast scan clock. This is a non-issue in your case because your board is simultaneously sampling.

Hope that helps.

Ron