National Instruments DAQCard-1200 - Manual (Page 44)

Chapter 3 Signal Connections
DAQCard-1200 User Manual 3-22 © National Instruments Corporation
DAQ and General-Purpose Timing Signal Connections
Pins 38 through 48 of the front connector are connections for timing
I/O signals. The DAQCard-1200 timing I/O uses two 82C53 counter/timer
integrated circuits. One circuit, designated 82C53(A), is used exclusively
for DAQ timing, and the other, 82C53(B), is available for general use.
You can use pins 38 through 40 and pin 43 to carry external signals for
DAQ timing in place of the dedicated 82C53(A). These signals are
explained in the next section, DAQ Timing Connections. Pins 41 through
48 carry general-purpose timing signals from 82C53(B). These signals are
explained in the
General-Purpose Timing Signal Connections section later
in this chapter.
DAQ Timing Connections
Each 82C53 counter/timer circuit contains three counters. Counter 0 on the
82C53(A) counter/timer, referred to as A0, is a sample-interval counter in
timed A/D conversions. Counter 1 on the 82C53(A) counter/timer, referred
to as A1, is a sample counter in controlled A/D conversions. Therefore,
counter A1 stops data acquisition after a predefined number of samples.
These counters are not available for general use.
Instead of counter A0, you can use EXTCONV* to externally time
conversions. Figure 3-12 shows the timing requirements for the
EXTCONV* input. A falling edge on the EXTCONV* initiates an
A/D conversion.
Figure 3-12. EXTCONV* Signal Timing
The external control signal EXTTRIG can either start a data acquisition
sequence or terminate an ongoing data acquisition sequence depending on
the mode—posttrigger or pretrigger. These modes are software selectable.
A/D Conversion starts within
125 ns from this point.
tw 250 ns minimum
tw
tw
EXTCONV*
VIH
VIL