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National Instruments Corporation E-1 NI 2501/2503 User Manual
E

Common Questions

This appendix addresses common questions you may have while using
your NI 2501 or NI 2503 switch card.
How fast can I scan with the NI2501 switch card?
The NI2501 can be scanned as fast as 250 Kscans/s. However, at faster
scan rates, the accuracy of the card declines due to the settling time of the
card. National Instruments recommends you set the scan rate to allow for
enough settling time to ensure the desired accuracy.
How fast can I scan with the NI2503 switch card?
The NI2 503 uses relays that have typical operate and release times of 3m s.
In addition to the operate and release times, the NI 2503 also requires from
0.5 to 1.0 ms for the relay contacts to stop bouncing. The combination of
these two times are known as debounce time. The default debounce time for
the NI2 503 switch card is set to just under 5 m s. This set tin g allows the
card to be scanned at 200 scans/s. However, if the software implements
break-before-make, this would reduce the scan rate to 100 scans/s.
Typically, when using a DMM to make accurate measurements, the DMM
requires hundreds of ms. Consequently, the DMM becomes the limiting
factor in determining the scan rate.
What should I do if the software detects the board but the switches do
not switch?
• Verify that the switches do not switch. Close your application program
and then launch the soft front panel as described in the Setup and Test
document you received with your kit. The soft front panel shows the
state of each relay or FET switch on the board. Try closing and opening
the switches.
• Also be sure to check the return codes of the NI-SWITCH operation to
ensure that there are no errors (negative value) or warnings (positive
value). You can also use the NI-Spy utility to check for error codes.