Pci0 Arbitration Scheme - Compaq AlphaPC 264DP Technical Reference Manual

PCI Devices
4.6.1.3 PCI SCSI Interface
The Adaptec AIC7895 is used as a bridge from PCI to SCSI. The AIC7895 has two
ultra SCSI ports and is packaged in a 208-pin plastic quad flat pack (PQFP). Both
ports support ultrawide SCSI devices. Note that parity must be disabled for the
SRAM that attaches to the AIC7895.
4.6.2 PCI1
Three PCI bus expansion slots are available on PCI1, with one slot shared with the
ISA. All three slots support 64-bit devices. Note that 3.3 V and +5 V are provided to
the appropriate PCI connector pins.
4.6.3 PCI Arbitration
Arbitration logic is implemented in the Pchips, and the scheme is flexible and soft-
ware programmable. Note, however, that Pchip1 handles the arbitration for PCI1, but
the TIGbus FPGA is the arbiter for PCI0. The arbitration logic in the southbridge and
in Pchip0 is disabled.

PCI0 Arbitration Scheme

The arbitration is controlled by the TIGbus FPGA, using a dual round-robin priority
scheme. There are two rings, A and B, with the scheme shown in Figure 4–3. Ring A
has a round-robin priority between Pchip0, SCSI controller, slot 0, and the desig-
nated device from ring B. Ring B has a round-robin priority between slot 2, slot 1,
the southbridge bridge request, and the southbridge USB request.
Figure 4–3 PCI0 Arbitration Scheme
Pchip0
Slot 1
Functional Description
4–10
SCSI
Controller
Ring A
Slot 0
Arbiter
Slot 2
Ring B Southbridge
Arbiter
Bridge
Southbridge
USB
12 February 1999 – Subject To Change