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The DMA operation of this IEEE-488 interface is quite transparent to users. Once
you select the DMA mode by calling INIT routine, all further data transfer proceeds
in this mode. Bowever, when you select block-transfer mode, the DMA channel 0
memory refresh may be held long enough to corrupt RAM memory content. Do not
use block-transfer mode unless you have confidence that the data transfer will be
completed within the time limits of memory refresh.
When using background operation, the CPU does not care about the data transfer. The
data string can be terminated by byte count only. The data format problem then must
be handled by users. It is safe to use DMA in single-byte-transfer and non-back-
ground operation mode (the default condition). The other modes can be used only
when you have solid understanding of the PC, 8237 DMA chip and your IEEE-488
devices. We do not recommend using these modes.
CHAPTER 5 ADVANCED PROGRAMM1N TECBNIQUES
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