National Instruments 700 Timing Connections

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

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 µs. The EXTCONV* input is a TTL-compatible signal.

tw 200 ns Minimum

A/D Conversion Starts Here

VIL

VIH

EXTCONV*

tint tint 10 µs 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