Chapter 3
Signal Connections
© National Instruments Corporation
3-7
DAQCard-500 User Manual
Timing Connections
Pins 20 through 26 and pin 11 of the I/O connector are for timing I/O signals. The
DAQCard-500 uses an MSM82C54 counter/timer integrated circuit. Counters 1 and 2 are
available at the I/O connector. Counter 0 is dedicated for data acquisition timing. Pin 11 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 in the Data Acquisition Counter and Timing
Connections section of this chapter. Pin 20 carries the 1 MHz square-wave oscillator signal.
This signal can be used as a timebase for the counter clock inputs. Pins 22 through 26 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 11, EXTCONV*, to externally time
conversions. If you need to program this chip directly, refer to the optional DAQCard-500
Register-Level Programmer Manual for the programming sequence needed to enable this input.
Figure 3-4 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 20 µs. The EXTCONV* input is a TTL-compatible signal.
tw 200 ns Minimum
A/D Conversion Starts Here
V IL
VIH
tw
EXTCONV*
tint
tint 20
µ
s Minimum
(A/D interval)
Figure 3-4. 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 CLK and OUT signals for counter 1 and GATE,
CLK, and OUT signals for counter 2 of the MSM82C54. 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 measurement. For these applications, CLK and GATE signals are sent to
the counters and the counters are programmed for various operations. The only exceptions are
counter 0, which has an internal 1 MHz clock and its gate is always enabled, and counter 1,
which has its gate always enabled.