User guide

10.3.1 Arbitrating the VMEbus
10.3.1.1 Requesting the VMEbus
Three arbitration schemes — priority, round-robin, and single-level — are
achieved by a combination of setting the arbiter/requester configuration register
(VIC_ABR, offset 0xB0) and using the VMEbus request lines. See Table 10–7 and
Figure 10–11.
The granting of ownership of the VMEbus to a master is passed down the
VMEbus along a daisy-chain. Because of this arrangement, the masters further
down the daisy-chain may be blocked by masters higher up the chain. This
problem (bus starvation) can be minimized if the masters all implement a Fair
Request scheme. If any master does not obey the fairness scheme, it can starve
the masters further along the daisy-chain.
Under the Fair Request scheme, the Digital Alpha VME 4 system does not
request the VMEbus for the duration of a fairness timeout period, if any
other master is requesting the VMEbus. When the timeout period expires,
the Digital Alpha VME 4 system asserts its request regardless of other requests.
The fairness timeout period gives any other masters along the daisy-chain the
opportunity to win the VMEbus.
Figure 10–11 VIC Arbiter/Requester Configuration Register
31 07 06 05 04 03 00
ML013372
PRI(1)/RRS(0)VMEbus Arbitration
VMEbus Request Level
Don't Care
VME_IF_BASE+B0 :
Fairness Timeout
VIC_ARCR
0
08
10–18 VME Interface