When you implement a control system in LabVIEW there are some items you should consider in the implementation.

• Your control system will operate with a loop.  In the loop you will actually perform control actions that must be done repeatedly.  However, some items are best moved outside the loop.
• You need to reset instruments.  In the most basic control system you will have a measurement instrument (Data Acquisition Unit or DVM) and something like a power supply to drive the actuator.
• Reset both instruments outside of the loop.
• You may need to activate the output(s) of the power supply.
• Activate the power supply outside the loop.
• In some situations you might have to provide power to a sensor or circuit in which the sensor is embedded.
• Turn on the power supply (activate it) for sensor circuitry outside the loop.
• If these are all GPIB/IEEE-488 instruments, in LabVIEW an error signal is developed.  For those actions taken outside the loop run the error signal from the first GPIBWrite function to other functions in an unbroken progression.  Do not run the error signal into the loop, and then try to bring it back out.
• Inside the loop, try to observe the following.
• If you have a single unbroken string of error messages, then you can control the sequence in which things happen.  Within the loop, a good sequence is the following:  (And a careful observer will note that the sequence here is the sequence in which things seem to take place within the control loop on the block diagram of the control system.
• Take the measurement.  In other words, string together all of the operations needed to take the measurement.  You should wire the error signal from outside the loop to the first GPIBWrite in this sequence.  In other words, the error signal will be wired from outside the loop to a GPIBWrite inside the loop.  Once inside, you should not try to bring it back to a GPIB block outside the loop.
• Compute the control effort.
• Generate the control effort using the power supply.  Use whatever GPIB blocks are necessary to produce the voltage you need.  Wire the last error from the measurement string to the first GPIB block in this set and wire them in sequence.
• Finally, add whatever else is necessary, including opening a file, writing data to a file and closing the file.  Remember that there is another error string for those operations, and you should not wire the error string from the GPIB blocks to the file operations blocks.