ADLINK ACL-8112 Series - Manual (Page 51)

Operation Theory
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The A/D data should now be transferred into the PC's memory for further
processing. The ACL-8112 provides three data transfer modes that allow
users to optimize their DAS system. Refer to section 5.1.3 for data transfer
modes.
5.1.2 A/D Trigger Modes
The ACL-8112’s A/D conversion can be triggered by either an Internal or
External trigger source. JP5 is used to set either internal or external triggers.
Please refer to section 2.8 for details. Whenever an external source is set,
the internal sources are disabled.
The two internal sources can either be software or timer pacer triggered
which is controlled by the A/D operation mode control register (BASE+11). In
total there a three different trigger sources possible to the ACL-8112. The
different trigger conditions are specified below:
Software trigger
This trigger source is software controllable. That is, the A/D conversion is
initiated when any value is written into the software trigger register
(BASE+12). This trigger mode is suitable for low speed A/D conversions.
Under this mode, the timing of the A/D conversion is fully controlled by the
software. However, it is difficult to have total control over a fixed A/D
conversion rate unless another timer interrupt service routine is used to
generate a fixed trigger rate.
Timer Pacer Trigger
An on-board 8254 timer/counter chip is used to provide a trigger source for
the A/D conversion at a fixed rate. Two counters are cascaded together to
generate a trigger pulse at precise intervals. Refer to section 5.4 for details
of the 8254 architecture. This mode is ideal for high speed A/D conversions.
It can be combined with DMA or interrupt data transfer. It's recommend that
this mode be used for applications that require a fixed and precise A/D
sampling rate.
External Trigger
Through pin-17 of CN3 (ExtTrig), the A/D conversion can also be performed
when a rising edge from an external source occurs. The conversion rate of
this mode is more flexible than the previous two modes, because users can
easily control an external device. An external trigger can be combine with
DMA transfer, interrupt data transfer, or even program polling data transfer.
Generally, the interrupt data transfer is used when external trigger mode is
used.