National Instruments DAQCard-1200 - Manual (Page 47)

Chapter 3 Signal Connections
© National Instruments Corporation 3-25 DAQCard-1200 User Manual
Figure 3-15. Interval-Scanning Signal Timing
Use the final external control signal, EXTUPDATE*, to externally control
updating the output voltage of the 12-bit DACs and/or to generate an
externally timed interrupt. There are two update modes—immediate update
and delayed update. In immediate update mode the analog output is
updated as soon as a value is written to the DAC. If you select the delayed
update mode, a value is written to the DAC; however, the corresponding
DAC voltage is not updated until a low level on the EXTUPDATE* signal
is sensed. Furthermore, an interrupt is generated whenever a rising edge is
detected on the EXTUPDATE* bit. The interrupt service routine can write
the next value to the DAC. When the EXTUPDATE* signal goes low, the
DAC will subsequently be updated. Therefore, you can perform externally
timed interrupt-driven waveform generation on the DAQCard-1200.
Notice that the EXTUPDATE* signal is level sensitive; that is, if you
perform writes to the DAC when EXTUPDATE* is low, the DAC is
updated immediately. Also notice that when EXTUPDATE* is low, the
signal is susceptible to noise caused by switching of other lines. Because
the signal is TTL-compatible, the lower noise margin for logical 0 can
result in transitions to a logical 1 due to noise and thereby generate false
interrupts. Therefore, the width of EXTUPDATE* pulse should be as short
as possible and greater than 50 ns.
Figure 3-16 illustrates a waveform generation timing sequence using the
EXTUPDATE* signal and the delayed update mode. The DACs are
updated by a high level on the DAC OUTPUT UPDATE signal, which in
OUTB1
EXTCONV*
CONVERT
GATE
tw = 50 ns
td = 50 ns
ADC CH CH1 CH0 CH1 CH0