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3.3
Digital Input
3.3.1
Digital Input Acquisition Methods
The device supports both instant (software-timed) and buffered (hardware-timed) dig-
ital input acquisitions.
3.3.1.1
Instant (Software-Timed) Digital Input Acquisition
With instant acquisition, the software controls the rate and time of acquisition, which
is thus also called software-timed acquisition. Whenever the software sends a read
command, the current status of digital input ports is returned as shown in Figure 3.15.
Figure 3.15 Instant (software-timed) digital input acquisition.
The advantage of instant acquisition is low latency. It is typically used for reading a
single sample of digital input.
3.3.1.2
Buffered (Hardware-Timed) Digital Input Acquisition
With buffered acquisition, a hardware signal called sample clock controls the rate and
time of acquisition as shown in Figure 3.16. The sample clock can be generated
internally on the device or be provided externally. Refer to the device specifications
for supported sample clock sources and the maximum allowable frequency of sample
clock.
Figure 3.16 Buffered (hardware-timed) digital input acquisition.
The acquired samples are first accumulated in the onboard first-in-first-out (FIFO)
memory of the device, and then moved to the buffer in the PC by a direct memory
access (DMA) engine. A buffer is a block of memory in the PC for temporarily storing
the data to be transferred to the application memory. Because the data is moved in
large blocks instead of one point at a time, buffered acquisition typically allow much
higher transfer rates. Buffered acquisition is also called hardware-timed acquisition.
PCI-1716_Series User Manual
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