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Analog Input Features
2-11
Pacer Clock
In paced mode, the pacer clock determines the conversion rate. The
following clock sources are available for paced mode conversions on
DAS-1800HC Series boards:
●
Software
— DAS-1800HC Series boards allow you to acquire single
samples under program control.
●
Hardware (internal clock source)
— The internal pace clock source
uses the onboard 82C54 counter/timer and a crystal-controlled 5MHz
time base. The internal pacer clock uses two cascaded counters of the
82C54 and is programmable between a maximum allowable rate of
333kHz and a minimum available rate of 0.0012Hz. When not used to
pace the analog input, the internal clock source can serve to pace
other events such as the digital I/O and analog outputs through the use
of interrupts.
●
Hardware (external clock source)
— The external pacer clock
source must be an externally applied TTL-compatible signal attached
to the DI0/XPCLK pin (B39) of the main I/O connector, J1. The
active edge for this clock is programmable.
An external clock source is useful if you want to pace at rates not
available with the 82C54 counter/timer, if you want to pace at uneven
intervals, or if you want to pace on the basis of an external event. An
external clock also allows you to synchronize multiple boards with a
common timing source.
Note:
The ADC acquires samples at a maximum of 333 ksamples/s (one
sample every 3.0µs). If using an external clock, make sure that it does not
initiate conversions at a faster rate than the ADC can handle.
If acquiring samples from multiple channels, the maximum sampling rate
for each channel is equal to 333 ksamples/s divided by the number of
channels.
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