Appendix B
Programmable Configurations
© National Instruments Corporation
B-7
VME-MXI-2 User Manual
VMEbus Requester
Request Level
The VME-MXI-2 uses one of the four VMEbus request levels
(0 to 3) to request use of the VME Data Transfer Bus (DTB). The
VME-MXI-2 requests use of the DTB whenever an external MXIbus
device attempts a transfer that maps into the VMEbus mainframe.
The VME-MXI-2 uses VMEbus request level 3 in its factory-default
setting. This is suitable for most VMEbus systems. However, you can
change the VME-MXI-2 to use any of the other three request levels
(0, 1, or 2) by writing to the EEPROM. You may want to change
request levels to change the priority of the VME-MXI-2 request signal.
Fair Request
The VME-MXI-2 is always a Release On Request requester. However,
you can configure whether the VME-MXI-2 acts as either a fair or
unfair requester on the VMEbus. By default, the VME-MXI-2 is a fair
requester. For more information on the different types of requesters,
refer to the VMEbus specification.
To change the VMEbus requester type of the VME-MXI-2, write the
EEPROM byte at offset 20B5 hex from the VME-MXI-2 base address.
The following table gives the value that should be written for the
corresponding requester type. The values shown in the table are
hexadecimal.
Bus Request Level
Fair Requester
Unfair Requester
Bus Request Level 3
07 (default)
17
Bus Request Level 2
06
16
Bus Request Level 1
05
15
Bus Request Level 0
04
14
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