User manual

Chapter 7 VXIplug&play for the VXI-MXI-2
VXI-MXI-2 User Manual 7-4 © National Instruments Corporation
These controls are necessary if you change the amount of DRAM
installed on the VXI-MXI-2. The amount of memory you set with
the Requested Memory control should match the amount of DRAM
installed on the VXI-MXI-2. If no DRAM is installed, you should set it
to 16 KB. Notice that the smallest valid amount in A32 space is 64 KB.
Caution: If you install DRAM into the VXI-MXI-2, do not attempt to use the
first 4 KB of memory space. This 4 KB space maps to the registers on
the VXI-MXI-2 and does not access onboard DRAM. Accessing this
region will cause your VXI-MXI-2 to behave incorrectly.
If you do not want to lose 4 KB of DRAM you can get around this
limitation by setting the Requested Memory control to double the
amount that is installed on the VXI-MXI-2, since the DRAM is aliased
throughout the remainder of the requested memory space. The DRAM
should then be accessed in the upper half of the requested memory
space.
A16 Write Post and A24/A32 Write Post
The VXI-MXI-2 can increase performance with its capability to post
write cycles from both the MXIbus and the VXIbus. Write cycles
should be posted only to devices that cannot return a BERR signal,
because the BERR will not be reported to the originating master. Use
the control appropriate for either A16 write posting or A24/A32 write
posting. For either control, the options are Enabled and Disabled. By
default, both options are disabled.
The A16 Write Post control affects only write cycles that map through
the Extender A16 window from the VXIbus to the MXIbus and vice-
versa. A16 write cycles in VXI configuration space are never posted
regardless of the setting of this control.
The A24/A32 Write Post control affects write cycles that map through
the Extender A24 window and Extender A32 window from the VXIbus
to the MXIbus and vice-versa. This control also affects write cycles to
the VXI-MXI-2 module via its requested memory space from both the
VXIbus and the MXIbus. For more information on the A16, A24, and
A32 windows, refer to Chapter 5, Register Descriptions.