I/O Performance Issues
6-28
example, if you are reading 32-bit integers from the host, you need to make
sure the host file contains the data in the correct byte order. Other than
correct byte order, there are no special format or data type requirements for
data to be transferred between the host and the target.
While you are developing a program, you can use HST objects to simulate
data flow and to test changes made to canned data by program algorithms.
During early development, especially when testing signal processing
algorithms, the program would explicitly use input channels to access data
sets from a file for input for the algorithm and would use output channels to
record algorithm output. The data saved to a file with the output host channel
can be compared with expected results to detect algorithm errors. Later in the
program development cycle, when the algorithm appears sound, you can
change the HST objects to PIP objects communicating with other threads or
I/O drivers for production hardware.
6.6.1 Transfer of HST Data to the Host
While the amount of usable bandwidth for real-time transfer of data streams
to the host ultimately depends on the choice of physical data link, the HST
Channel interface remains independent of the physical link. The HST
Manager in the configuration allows you to choose among the physical
connections available.
On the C55x and C6000 platforms, the host PC triggers an interrupt to
transfer data to and from the target. This interrupt has a higher priority than
SWI, TSK, and IDL functions. The actual ISR function runs in a very short
time. Within the idle loop, the LNK_dataPump function does the more time-
consuming work of preparing the RTDX buffers and performing the RTDX
calls. Only the actual data transfer is done at high priority. This data transfer
can have a small effect on real-time behavior, particularly if a large amount of
LOG data must be transferred.
6.7 I/O Performance Issues
If you are using an HST object, the host PC reads or writes data using the
function specified by the LNK_dataPump object. This is a built-in IDL object
that runs its function as part of the background thread. On the C55x and
C6000 platforms, the actual data transfer occurs at high priority.
The polling rates you set in the LOG, STS, and TRC controls do not control
the data transfer rate for HST objects. Faster polling rates actually slow the
data transfer rate somewhat because LOG, STS, and TRC data also need to
be transferred.
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