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Chapter 5 Programming Considerations
© National Instruments Corporation 5-3 VXI-MXI User Manual
The recommended way to set up your system is to fill up Level 1 MXIbus links before adding
additional levels. System performance decreases as the number of levels in the system increases
because each level requires additional signal conversion. Also keep in mind these three basic
rules for VXI-MXI installation as you decide where to install your VXI-MXI interfaces:
1. The VMEbus bus timeout unit must be on a VXI-MXI.
2. Multiple VXI-MXIs in a mainframe must be in adjacent slots.
3. If in interlocked bus mode, all VXI-MXIs must be the highest priority VMEbus requesters in
that frame, with the possible exception of one device in the MXIbus link.
The address mapping windows on the VXI-MXI can be configured to have a Base/Size format or
a High/Low format. The CMODE bit in the MXIbus Control Register selects which format the
mapping windows use.
Base/Size Configuration Format
Each address mapping window on a MXIbus interface has Base and Size parameters associated
with it when the CMODE bit in the MXIbus Control Register is cleared. The Base bits define
the base address for the window, and the Size bits indicate the number of Base bits that are
significant. Replacing the insignificant bits with zeros gives the actual base address of the
window. In other words, the Base and Size define a range of addresses that are in the window.
A Direction bit is also included to indicate whether the defined range of addresses are mapped
into or out of the VXIbus mainframe.
Table 5-1 shows which bits are compared for each Size setting and the resulting address range in
hex if Base is set to 0 and hex 55. Figure 5-3 further illustrates the number of bits of the Base
that are compared for each Size value. Notice that if Size = 0, no bits are compared. Figure 5-4
shows the address range allocation for different Size values.
Table 5-1. Base and Size Combinations
Range Range
Size Base7 Base6 Base5 Base4 Base3 Base2 Base1 Base0 for 0 for 55
7******* 0 to 154 to 55
6****** 0 to 354 to 57
5***** 0 to 750 to 57
4 * * * * 0 to F 50 to 5F
3 * * * 0 to 1F 40 to 5F
2 * * 0 to 3F 40 to 7F
1 * 0 to 7F 00 to 7F
0 0 to FF 00 to FF