2-14
Functional Description
Hardware Gates
A hardware gate is an externally applied digital signal that determines
whether conversions occur. You connect the gate signal to the IP1 / TRIG
pin (pin 25) on the main I/O connector. DAS-800 Series boards support a
positive gate only. Therefore, if the hardware gate is enabled and the
signal to IP1 / TRIG is high, conversions occur; if the signal to IP1 /
TRIG is low, conversions are inhibited.
Note:
You cannot use the hardware gate with a hardware trigger.
However, the gate signal itself can act as a trigger. If the gate signal is low
when the software starts the analog input operation, the board waits until
the gate signal goes high before conversions begin.
When using the hardware gate, the way conversions are synchronized
depends on whether you are using a hardware external clock or a
hardware internal clock, as follows:
●
External clock - The signal from the external clock continues
uninterrupted while the gate signal is low; therefore, conversions are
synchronized to the external clock.
●
Internal clock - The 8254 does not count while the gate signal is low.
Whenever the gate signal goes high, the 8254 is loaded with its initial
count value and starts counting; therefore, conversions are
synchronized to the gate signal.
Figure 2-5 illustrates how to use the hardware gate with both an external
clock and an internal clock.
