Chapter 6
NI-VXI Configuration Utility
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National Instruments Corporation
6-9
disabled, you can lower the MXI Bus Timeout because there will be no
delay due to the inward cycles being retried.
Note
The PCI-MXI-2 has a limit on the number of automatic retries it will perform on any
one cycle. If the limit is exceeded and the PCI-MXI-2 receives another retry, it will pass a
retry back to the MXIbus even though the Enable MXI-2 Auto Retry option is selected.
A24/A32 Write Posting
This field determines whether to enable write posting for incoming slave
accesses to A24/A32 VXI/VME shared RAM. By default this option is
disabled.
Enabling write posting will increase the throughput of your inward cycles.
However, you should not enable write posting unless the destination of your
inward A24/A32 cycles is onboard RAM, because cycles to onboard RAM
will always complete successfully.
PCI Bus
The following paragraphs describe the options for the PCI Bus portion of
this editor.
User Window and Driver Window
The PCI-MXI-2 driver requires the use of two PCI windows: a user
window and a driver window. Calling the
MapVXIAddress()
function
allocates regions of the user window to your application.
VXIpeek()
and
VXIpoke()
accesses are performed through this window. NI-VXI uses the
driver window to perform high-level functions such as
VXIin()
and
VXIout()
, and to access registers on the PCI-MXI-2 and
VXI/VME-MXI-2.
The windows are mapped to PCI base address registers and determine the
amount of PCI memory space the PCI-MXI-2 requests from the PCI system
during initialization.
User Window Size
The amount of space you can allocate for the user window is system
dependent. You can use the Size option to select the size of the user window
(minimum of 4 KB, maximum of 2 GB). The more you increase the size of
the user window, the larger the window you can map in
MapVXIAddress()
.
