User`s guide

I/O Performance Issues
Input/Output Overview and Pipes 6-15
6.4.1 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 Tool allows you to choose among the physical
connections available.
The actual data transfer to the host occurs within the idle loop on the C54x
platform, running at lowest priority.
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.
6.5 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. Since background
threads have the lowest priority, software interrupts and hardware interrupts
can preempt data transfer on the C54x platform, whereas 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.