Triggering and reading on two HP/Agilent 34401A DMMs at same time

Question

Hi folks,
I'm VERY new to LabVIEW and need some help with a measurement.  I'm using LV 8.2 with all equipment on GPIB.  I have two 34401A multimeters, triggered externally using an HP 33120A function generator.   I've downloaded the example vi's and drivers and everything works fine.  My problem is with the speed of the triggering and the synchronization between the two multimeters on the same trigger.  I've read the manual and for this multimeter, it claims it can be triggered and read over GPIB at 1kHz (display off, auto range off, delay= 0, 4.5 digits).  I'm finding it is nowhere near this speed.  The fastest I have successfully triggered and recorded the data was at 100 Hz, anything higher and it no longer stays in sync with the trigger (i.e. for 200 measurements at 100 Hz, I'm getting all 200 measurements, but over 3-5 seconds).  The trigger speed is the first problem.  The second problem is getting the two meters to take a reading at the same time.  One meter always lags the other (or vice versa) by 1 to 3 readings.  My goal is to continuously (for about 60 seconds) read both multimeters as fast as possible and at the same time (250-500 Hz would be fantastic) and write it to a file.  Can anyone give me some advice on this?  Is a software trigger the way to go?  Is there some limitation with GPIB that it cannot read/write to two devices this fast? 
Thanks!
P.S. I've been experimenting mostly with the example code provided by NI and a .vi posted in this forum thread (http://sine.ni.com/niforum/niforumforumDU=http://forums.ni.com/ni/board/messageboard.id=170&message....) hp34401a_-_testing.vi from Kunal.  If anyone could share some other example vi's it would be very helpful!

Answer 1

Hi plasma420,
I should have read your post more carefully - was thinking it was a 1 sec acquisition, not 1 minute!
Getting your LabVIEW app to reliably respond to a measurement (detect/read) every 2 to 4 ms is going to be tough - especially if in a multi-tasking OS environment.  On a 2GHz box running a normal Windows XP or 2K install,  I suspect you could see an occasional "hiccup" even at 100Hz.  A bit of searching the forum turned up this old-but-still-valuable (even entertaining) post related to improving determinism of DAQ applications on a PC.
Observation: The listed max measurement-rate of the 34401 (1000Hz) may assume the GPIB-bus is dedicated to one instrument - the requirement to read from two instruments sequentially, may necessarily increase time spent servicing each trigger - decreasing maximum theoretical throughput.
The first rule for "high-speed" multi-sample DAQ - especially under Windows - is to make it a hardware task instead of a software loop;  the 34401 doesn't seem well suited here.  I hate to suggest this, but... have you considered purchasing one of NIs multi-channel DAQ devices?  With a $360 PCI 6010 + cable and break-out box, you could have one of the LabVIEW DAQ examples running in ~ 30 min.  The 6010 was the cheapest 16-bit board I could find, ask your sales-rep for other alternatives if you go this route...
Cheers!
"Inside every large program is a small program struggling to get out." (attributed to Tony Hoare)