ADLINK Technology NuDAQ PCI-9810, User Manual

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42 Operation Theory
Storing the data from host memory and into a hard disk or other
storage devices should also be considered. The burst data rate of
current HDD technology could be between 90 and 80MB/s.
However in reality, the HDD effective bandwidth is usually less
than 10MB/s, especially when the HDD seek time is longer, for
example, 20ms, the FIFO is already full and the acquired data
could not be continuous.
Another limitation comes from the OS and host memory size.
Under DOS environment, the maximum memory size that can be
allocated is less than 640K except that you use extended memory
managing software. Under Windows-95 or NT, it is relatively
difficult to get a continuously large memory size such as 64MB. It
is possible to allocate a large memory size by keeping the memory
‘clean’. The PCI bus-mastering DMA controller of PCI-9812 needs
continuous memory to store the AD data.
Summary
So far the AD data transferring, the maximum sampling rate, and
the maximum continuous data length at the sample time have
been discussed. Below is a summary of the steps discused:
1. Calculate the total data throughput;
2. Check if the total data throughput is higher than the
effective PCI bandwidth in your system;
3. If it is higher, then the maximum data length will be
limited to 16K samples for two channels and 8K samples
for four channels;
4. If it is lower, then the maximum total data size is 64M
bytes, which comes from the limitation of host memory;
5. To acquire the AD data continuously (ie the data length
is infinite); the ‘double-buffered’ software can be used.
However, the CPU computation bandwidth still needs to
be calculated to check if the continuity is possible.