digital trigger for data acquisition in LabVIEW 5.0.1

[hans 264]

I tried to start a data acquisition with an digital trigger-pulse and set the trigger-type-input of the function "AI Start" to "digital A". But when I debugged my VI, I noted that the process waited in "AI read" for the start-pulse and not in "AI start"....
Does this mean that the acquisition started with "AI start" and the data were only read (from buffer) after the pulse occured?
What can I do to start the acquisition itself with the digital-trigger-pulse?

hans

[Gérard]

I guess that your data acquisition never started for lack od aporper TTL
triiger sig and that you tried to read a number of points from the
buffer that were not available in hte first place. If yu have a timeout,
the vi will exit after a while..
You never get stuck in AIStart but in AI read. That is the reason by the
way that NI introduced a DAQ event activated after a number of points are
reached in the buffer and thus avoid waiting for these data to be present
and cater to other tasks in hte meantime.
It may also occur that your trigger signal cannot supply enough sink or
source current (typically 3.5 to 5mA on my E series cards) Best way is to
watch on a scope (vi why not) whether you signal gest attenuated when
wiring it to the Trigger pin.
Incidentally
watch your trigger unit if you plan to wire it to Gate for
counters. I got lots of problem with rebounds. Not the fault of NI, just
that their TTL hardware is fast on E series cards and picks bursts.

hans wrote:

> I tried to start a data acquisition with an digital trigger-pulse and
> set the trigger-type-input of the function "AI Start" to "digital A".
> But when I debugged my VI, I noted that the process waited in "AI
> read" for the start-pulse and not in "AI start"....
> Does this mean that the acquisition started with "AI start" and the
> data were only read (from buffer) after the pulse occured?
> What can I do to start the acquisition itself with the
> digital-trigger-pulse?
>
> hans

--
Regards
Gerard D'Ans (gerard.dans@laborelec.be)
Laborelec Rodestraat,125 B-1630 Linkebeek
tel 32 2 382 0568
fax 32 2 282 0241
ULB SMA (gdans@ulb.ac.be)
Blvd F D Roosevelt CP165 B-1050 Brussels
Tel 32 2 650 2515
http://www.ulb.ac.be/polytech/laborulb/index.htm

[Filipe A.]

Hans;

It's looking like your external digital pulse trigger is out of the TTL specifications, or you are having something else triggering your task.
I advise you to run the Labview shipping example named: "Continuous Acquisition and Graph (buffered) using Digital Triggering.vi". That VI does exactly what you need. If you still see the problem when running that example, the problem is either at your external pulse or at your board itself.
You can find that example by going to Search Examples->I/O Interfaces->Data Acquisition->Analog Input->Triggering an Acquisition->Triggering a Continuous Acquisition.
Hope this helps.
Filipe

[hans 264]

Filipe;

the triggering seems to work (since I only get data when the trigger-pulse occurs), but not in the way, I have imagined. Since the process waites in "AI read.vi" for the trigger-pulse (and not in "AI start.vi"), I guess that the acquisition runs the whole time and the pulse only starts the reading of the data (from buffer).

Is that right or should the process wait in "AI start.vi" until the trigger occure?

Hans