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2-16
Functional Description
Figure 2-4. Timing of Data Conversion Modes for Channels 4 to 7
Clock Sources
DAS-1800ST/HR Series boards provide two clocks: a pacer clock and a
burst mode conversion clock. In paced mode, the pacer clock works alone
to time interrupt-mode and DMA-mode operations, as shown in Figure
2-4. In burst mode and burst mode with SSH, the pacer clock and the
burst mode conversion clock work together to time interrupt-mode and
DMA-mode operations, also shown in Figure 2-4. These clock sources
are described in the following sections.
Pacer Clock
In paced mode, the pacer clock determines the conversion rate. In burst
mode and burst mode with SSH, the pacer clock determines the sample
rate (the rate at which a single channel is sampled). The following pacer
clock sources are available for DAS-1800ST/HR Series boards:
●
Software
— DAS-1800ST/HR Series boards allow you to acquire a
single sample under program control.
●
Hardware (internal clock source)
— The internal pacer clock
source uses the onboard 82C54 counter/timer and a crystal-controlled
5MHz time base. The onboard pacer clock uses two cascaded
counters of the 82C54. The maximum allowable rate is 333kHz for
DAS-1800ST Series boards and 100kHz for the DAS-1802HR; the
minimum available rate is 0.0012Hz. When not used to pace the
Pacer Clock
Paced Mode Conversions
Burst Mode Conversions
Burst Mode Conversions
(with SSH)
Burst Mode Conversion Clock
CH4
CH5
CH4 CH5 CH6 CH7
CH4 CH5 CH6
CH7
CH7
CH6
CH5
CH4
Hold
CH7
CH6
CH5
CH4
Hold
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