Appendix B
Programmable Configurations
VME-MXI-2 User Manual
B-6
© National Instruments Corporation
VMEbus Arbiter
Arbiter Type
You can configure the VME-MXI-2 as either a Priority or Round Robin
VMEbus arbiter. This setting is applicable only if the VME-MXI-2 you
are configuring is the first slot device. The default is Priority.
When configured for Priority arbitration, the VME-MXI-2 grants
the bus to the highest bus request level pending. In Round Robin
arbitration mode, the VME-MXI-2 grants the bus to the next highest
bus request level after the level of the previous bus owner. This
effectively gives the same priority to each bus request level. Refer to
the VMEbus specification for more information on the different types
of arbiters.
Arbiter Timeout
An arbitration timeout feature is available on the VME-MXI-2 when
it is acting as the VMEbus arbiter. This feature applies only to a
VME-MXI-2 in the first slot. This feature is enabled by default.
The timer begins when the arbiter circuit on the VME-MXI-2 drives
one of the BGOUT lines on the backplane. If no device takes over the
bus within the timeout limit, the BGOUT is removed and the bus is
either idle or granted to another requester.
To change the VMEbus arbiter type of the VME-MXI-2, write the
EEPROM byte at offset 20B4 hex from the VME-MXI-2 base address.
The following table gives the value that should be written for the
corresponding arbiter type. The values shown in the table are
hexadecimal.
Timeout Status
Priority Arbiter
Round Robin
Arbiter
Arbiter Timeout Enabled
00 (default)
80
Arbiter Timeout Disabled
40
C0
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