National Instruments 700 - Manual (Page 33)

Signal Connections Chapter 3
DAQCard-700 User Manual 3-14 © National Instruments Corporation
Timing Connections
Pins 38 through 48 of the I/O connector are connections for timing I/O signals. The
DAQCard-700 timing I/O uses an MSM82C54 counter/timer integrated circuit. All three
counters of the MSM82C54 are available at the I/O connector. One of these counters, counter 0,
is used for data acquisition timing. Pin 40 carries an external signal, EXTCONV*, that can be
used for data acquisition timing in place of counter 0 of the MSM82C54. This signal is
explained under Data Acquisition Timing Connections. Pins 38 and 41 through 48 carry general-
purpose timing signals from the MSM82C54. These signals are explained under General-
Purpose Counter and Timing Signal Connections later in this chapter.
Data Acquisition Counter and Timing Connections
Counter 0 on the MSM82C54 counter/timer is used as a sample-interval counter in timed A/D
conversions. In addition to counter 0, you can use pin 40, EXTCONV*, to externally time
conversions. If you need to program this chip directly, refer to the optional DAQCard-700
Register-Level Programmer Manual for the programming sequence needed to enable this input.
Figure 3-7 shows the timing requirements for the EXTCONV* input. An A/D conversion is
initiated by a rising edge on the EXTCONV*. The data from this conversion is latched into the
FIFO memory within 10
µ
s. The EXTCONV* input is a TTL-compatible signal.
tw 200 ns Minimum
A/D Conversion Starts Here
V IL
VIH
tw
EXTCONV*
tint
tint 10 µ
s Minimum
(A/D interval)
Figure 3-7. EXTCONV* Signal Timing
Notice that EXTCONV* can only cause conversions to occur; you cannot use it as a monitor to
detect conversions caused by the onboard sample-interval timer.
General-Purpose Counter and Timing Signal Connections
The general-purpose timing signals include the GATE, CLK, and OUT signals for the three
MSM82C54 counters, except CLK of counter 0 is not available on the I/O connector. You can
use the MSM82C54 counter/timers for general-purpose applications such as pulse and square
wave generation; event counting; and pulse-width, time-lapse, and frequency measurement. For
these applications, CLK and GATE signals are sent to the counters, and the counters are