Motorola MVME1X7P - Manual (Page 107)

Functional Blocks
http://www.motorola.com/computer/literature 2-17
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VMEbus System Controller
With the exception of the optional SERCLK driver and the Power Monitor,
the chip includes all the functions that a VMEbus system controller must
provide. The system controller is enabled/disabled with the aid of an
external jumper (J1), the only jumper required in a VMEchip2-based
VMEbus interface.
Arbiter
The arbitration algorithm used by the chip arbiter is selected by software.
All three arbitration modes defined in the VMEbus Specification are
supported: Priority (PRI), Round-Robin-Select (RRS), as well as Single
(SGL). When operating in the PRI mode, the arbiter asserts the BCLR line
whenever it detects a request for the bus whose level is higher that the one
being serviced.
The chip includes an arbitration timer, preventing a bus lockup when no
requester assumes control of the bus after the arbiter has issued a grant.
Using a control bit, this timer can be enabled or disabled. When enabled,
it assumes control of the bus by driving the BBSY signal after 256
µ
secs,
releasing it after satisfying the requirements of the VMEbus specification,
and then re-arbitrating any pending bus requests.
IACK Daisy-Chain Driver
Complying with the latest revision of the VMEbus specification, the
System Controller includes an IACK Daisy-Chain Driver, ensuring that
the timing requirements of the IACK daisy-chain are satisfied.
Bus Timer
The Bus Timer is enabled/disabled by software to terminate a VMEbus
cycle by asserting BERR
∗
if any of the VMEbus data strobes is maintained
in its asserted state for longer than the programmed time-out period. The
timeout period can be set to 8, 64, or 256 secs. The bus timer terminates an
unresponded VMEbus cycle only if both it and the system controller are
enabled.