User manual

Appendix B Programmable Configurations
VXI-MXI-2 User Manual B-6 © National Instruments Corporation
VMEbus Arbiter
Arbiter Type
You can configure the VXI-MXI-2 as either a Priority or Round Robin
VMEbus arbiter. This setting is applicable only if the VXI-MXI-2 you
are configuring is the first slot device. The default is Priority.
When configured for Priority arbitration, the VXI-MXI-2 grants the bus
to the highest bus request level pending. In Round Robin arbitration
mode, the VXI-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 VXI-MXI-2 when it
is acting as the VMEbus arbiter. This feature applies only to a VXI
Slot 0VXI-MXI-2. This feature is enabled by default.
The timer begins when the arbiter circuit on the VXI-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 VXI-MXI-2, write the
EEPROM byte at offset 20B4 hex from the VXI-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