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Analog Output Features
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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
family, DMA is directed by one or more 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-1800HC 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 singly 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 software
driver automatically uses the FIFO to buffer the samples while the
DMA channel is being re-programmed for another address. In most
situations, this FIFO buffering capability allows you to acquire and
load 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 operation in the Windows
Enhanced Mode, you should use dual-channel DMA to acquire data
reliably at maximum board speeds.
Analog Output Features
The analog output section of DAS-1800HC Series boards consists of two
DACs with 12-bit resolution. Both DACs have a fixed voltage range of
±10V, and they power up to 0V at reset. The two DACs have a capacitive
load drive of 100µF and an output current drive of ±5mA.
The analog output can be “paced” with interrupts generated by the
onboard pacer clock when the analog inputs are either disabled or timed
by an external pacer clock. Single values can be written to the DACs.
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