Data Translation DT9800 Series, Руководство пользователя

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Chapter 2
36
Data Transfer
The module packs two bytes into each transfer to the host computer.
Even samples (corresponding to entries 0, 2, 4, and so on, in the
channel-gain list) are packed into the low bytes; odd samples
(corresponding to entries 1, 3, 5, and so on, in the channel-gain list)
are packed into the high bytes.
DT9800 Series function modules contain a 2048-sample FIFO. During
a continuous analog input operation, the hardware interrupts the
firmware on the module when the FIFO is half full. The module then
transfers 2048 samples to a circular buffer, which is dedicated to the
hardware, in the host computer.
The DT9800 Series Device Driver accesses the hardware circular
buffer to fill user buffers that you allocate in software. Keep the
following recommendations in mind when allocating user buffers for
continuous analog input operations on the DT9800 Series:
• Allocate a minimum of three user buffers.
• Specify a buffer size at least as large as the sampling rate; for
example, if you are using a sampling rate of 100 kSamples/s
(100 kHz), specify a buffer size of 100,000. The minimum buffer
size that you should specify is 256 samples.
DT9800-EC-I Series DT9801-EC-I
DT9802-EC-I
FFFh
(plus full-scale)
000h
(minus full-scale)
DT9803-EC-I
DT9804-EC-I
FFFFh
(plus full-scale)
0000h
(minus full-scale)
Table 7: Overrange SIgnal Values (cont.)
Function
Module Series
Module
Name
Above-Range
Signals
Below-Range
Signals