Run voltage measurement on two NI 9205 at the same time cause error

[ping2002]

I have two NI 9205 on a CompactDAQ chassis cDAQ-9172, one 9205 is used to measure the resistive sensor voltage, the other one is used to measure the sensor current by measuring the voltage drop on a resistor in series with the sensor. I created SubVIs from DAQ assistant to measure the sensor voltage and sensor current repectively, the SubVIs run correctly. Then I put the two SubVIs in a another VI, this VI run the two SubVIs in parallel, then divide the voltage by current to get the sensor resistance. 

The problem  is that, when I run the parent VI in normal speed, only the current SubVI gives the correct output, while the voltage SubVI gives output "0", which is the default value to floating variable. Seemed that the voltage SubVI is not running at all. The parent program does not give any error message.  In the debug mode, the parent VI runs fine, while the speed is much slower, I can see that the current SubVI runs first, then the Voltage SubVI runs, they all produce the correct output. 

So it seemed that, the two analogue voltage input measurement can not take place at the same time. This is weird, the current and voltage measurement are on different NI 9205, they are not using the same ADC, so there should be no hardware conflict. 

Does anyone have a clue to overcome this error? It is important for me to measure the current and voltage for the sensor at the same time, since the resistance is changing over the time. 

thanks

Ping

Message Edited by ping2002 on 01-19-2009 05:45 PM

[MarkGrot]

You can only have one analog input task running at a time on a cDAQ-9172 chassis.  This means you cannot use two DAQ Assistants to create separate voltage and current tasks.  You can, however, add current channels to your voltage task (just use add channels in the Assistant).  When you add the current channels to your Assistant task, set the resistor location to external and fill in the value.  That should scale your current channels for you.

As to why you aren't seeing errors now, my guess is that you are dropping errors somewhere.

Also, the 9205 is a multiplexed device.  This means all channels use the same ADC.