National Instruments 321906B-01 用户手册

Chapter 2

NI 2501/2503 Operation

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 National Instruments Corporation

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Routing the external trigger from the front connector to the PXI trigger 
lines gives you the capability to trigger multiple switch cards with only one 
board connected to the external trigger from the measurement device. 

All external trigger lines are compatible with TTL voltage levels and are 
edge sensitive. 

The external trigger from the front connector requires a minimum pulse 
width of 500 ns. The line has a low pass filter to prevent false triggering. 
The minimum pulse width from the PXI TTL triggers and PXI star trigger 
is 70 ns.

The NI 2501/2503 can generate a scanner advanced trigger to indicate 
when the switch card is set up and ready to take measurements. Using the 
driver software, you can configure the switch card to route the scanner 
advanced (SCANADV) trigger to any PXI TTL trigger line, the PXI star 
trigger, or the SCAN_ADV pin on the front connector. You can configure 
the switch card to generate the SCANADV trigger when a relay has settled 
(or debounced). 

All SCANADV trigger lines are compatible with TTL voltage levels.

Because the NI 2501/2503 has open collector drives on the PXI TTL trigger 
lines, you can have multiple switch cards using the same trigger line in the 
multicard SCANADV mode. 

SCANADV has two modes of operation. In its default mode, the 
SCANADV trigger asserts for 1 µs after the relay has debounced. You 
can also configure the switch card for handshaking mode, in which the 
SCANADV trigger goes high after a relay settles, and the SCANADV 
trigger goes low after the external trigger input asserts. Use this 
handshaking mode for multicard scanning.