SBS Technologies 466-1, Hardware Manual

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Model 466-1 Adapter
8 Adapter Functions
The priority arbiter must assert BCLR when a bus master of higher priority
than the one in control of the bus initiates a request. When BBSY is asserted
and a request is pending, the arbiter will drive BCLR if the pending request is
of higher priority than the bus grant of the previous arbitration. Although the
current bus master is not required to relinquish control of the bus in any
prescribed time limit, it can continue transferring data until it reaches an
appropriate stopping point.
A round-robin arbiter gives equal priority to all bus request levels. It grants
control of the bus on a rotating basis. Upon release of the bus, the arbiter steps
one level and tests for an active request and asserts a bus grant. If no request
is active, it continues stepping through the levels until a request is found.
The RRS arbiter can drive the BCLR signal. In RRS mode BCLR is asserted
whenever a master requests the bus on a level other than the last one granted.
It does not assert BCLR if a master on the same level requests the bus.
1.6 Adapter CSR
Access from the VMEbus to the adapter Control and Status Registers ( CSR )
requires a minimum of 32 bytes of VMEbus I/O space. The SBus has a fixed
window size of 64K to access the adapter's
CSR . Only the first 16 bytes of
this space are actually used.