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The local bus to VMEbus requester in the VMEchip2 implements a
fair mode. By setting the LVFAIR bit, the requester refrains from
requesting the VMEbus until it detects its assigned request line in
its negated state.
The local bus to VMEbus requester attempts to release the VMEbus
when the requested data transfer operation is complete, the DWB
pin is negated, the DWB bit in the LCSR is negated and the bus is
not being held by a lock cycle. The requester releases the bus as
follows:
1. When the chip is configured in the release-when-done (RWD)
2. When the chip is configured in the release-on-request (ROR)
To minimize the timing overhead of the arbitration process, the
local bus to VMEbus requester in the VMEchip2 executes an early
release of the VMEbus. If it is about to release the bus and it is
executing a VMEbus cycle, the requester releases BBSY before its
associated master completes the cycle. This allows the arbiter to
arbitrate any pending requests, and grant the bus to the next
requester, at the same time that the active master completes its
cycle.
VMEbus to Local Bus Interface
The VMEbus to local bus interface allows an off-board VMEbus
master access to onboard resources. The VMEbus to local bus
interface includes the VMEbus slave, write post buffer, and local bus
master.
mode, the requester releases the bus when the above
conditions are satisfied.
mode, the requester releases the bus when the above
conditions are satisfied and there is a bus request pending on
one of the VMEbus request lines.
Functional Blocks
2-9
2
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