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Chapter 6
NI-VXI Configuration Utility
Request Level
The VXI/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
VXI/VME-MXI-2 requests use of the DTB whenever an external MXIbus
device, such as a PCI-based computer with a PCI-MXI-2 interface,
attempts a transfer that maps into the VXI/VMEbus mainframe.
The VXI/VME-MXI-2 uses VMEbus request level 3 by default, as required
by the VXIbus specification. This is suitable for most VXIbus systems.
However, you can change the VXI/VME-MXI-2 to use any of the other
three request levels (0, 1, or 2) by changing the setting of the Request
Level control. You may want to change request levels to change the priority
of the VXI/VME-MXI-2 request signal. For more information, refer to the
VMEbus specification.
VXI/VME Fair Requester
The VXI/VME-MXI-2 is always a Release On Request requester.
However, you can configure whether the VXI/VME-MXI-2 acts as either
a fair or unfair requester on the VXI/VMEbus. By default, the Operate as
Fair Requester checkbox is enabled, signifying a fair requester. For more
information on the different types of requesters, refer to the VMEbus
specification.
Arbiter Timeout
An arbitration timeout feature is available on the VXI/VME-MXI-2 when
it is acting as the VMEbus arbiter. This feature applies only to a VXI Slot 0
(VMEbus System Controller) VXI/VME-MXI-2. By default this option is
enabled.
The timer begins when the arbiter circuit on the VXI/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.
© National Instruments Corporation
6-25
PCI-MXI-2 for Linux
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