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Functional Description
Data Transfer Modes
Using the provided software, you can transfer data from
DAS-1800ST/HR Series boards to the computer using the following data
transfer modes:
●
Interrupt
— You can program the board to generate an interrupt for
events such as
FIFO Half Full
or
FIFO Not Empty
. FIFO Half Full
occurs after the FIFO accumulates 512 A/D samples for transfer to
computer memory. FIFO Not Empty occurs anytime the FIFO buffer
contains data.
An interrupt occurs in the background, allowing the CPU to execute
other instructions. The interrupt level is software-selectable.
Unpredictable interrupt latencies in the Windows environment tend to
make maximum board speeds unachievable in interrupt mode. When
in the Windows environment, you are advised to use single- or
dual-channel DMA instead of the interrupt transfer mode.
●
DMA
— DMA is a method of bypassing the CPU to transfer data
directly between an I/O device and computer memory. In the IBM PC
AT family, DMA is directed by the DMA controllers and can run in
the background while the CPU is executing other instructions. The
ability to run independent of the CPU and at high-transfer rates
makes DMA an attractive method for transferring data in data
acquisition systems.
DAS-1800ST/HR Series boards use DMA channels 5, 6, and 7 to
perform single- or dual-channel DMA transfers of A/D data from the
board to memory. When you set up your configuration file, you can
specify these channels individually for single-channel DMA or in
pairs for dual-channel DMA.
Each DMA channel can transfer up to 65,536 A/D samples before it
has to be reprogrammed with a new memory address. When more
than 65,536 samples are required by an application, the FIFO
automatically buffers the samples while the DMA channel is being
reprogrammed for another address. In most situations, this FIFO
buffering capability allows you to acquire large amounts of
gap-free
data into multiple buffers at up to maximum board speed using a
single DMA channel.
Generally, if you are programming operations in Windows, you
should use dual-channel DMA to acquire data reliably at maximum
board speeds.
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