User`s guide

The XMI
2.1.7 Arbitration
The XMI protocol includes arbitration because, at any time, any or all
of the nodes may desire the use of the XMI. Arbitration determines
which node gains the XMI when more than one node requests the XMI
simultaneously.
Table 2–4 XMI Arbitration Lines
Name Use
XMI CMD REQ L Initiates XMI transactions
XMI RES REQ L Returns data
XMI GRANT L Indicates which node has been granted the XMI bus for
the next cycle
The VAX 6000 platform supports an XMI bus of 14 nodes. Arbitration
cycles occur in parallel with data transfer cycles, since the XMI has a set
of lines dedicated to arbitration. These lines are listed in Table 2–4.
When a node desires ownership of the bus, it asserts one of its two request
lines (XMI CMD REQ L or XMI RES REQ L) that are connected to the
central arbiter. The XMI CMD REQ L line is used by nodes to initiate
XMI transactions (that is, act as a commander) while the XMI RES REQ
L line is used by nodes to return data to a commander (that is, act as
a responder). The XMI arbiter maintains two independent round-robin
queues, one for each request type. The responder requests are given
higher priority than commander requests.
See Section 2.3 for more information on arbitration.
2.1.8 Bus Integrity
The XMI bus contains a number of features to enhance the integrity and
reliability of the bus:
• All bus information transfer lines are parity protected.
• Bus confirmation signals are ECC protected.
• XMI protocol permits detection and recovery of almost all single-bit
errors on the information transfer lines and bus confirmation signal
lines.
• XMI protocol defines timeout conditions that are used to detect
failures.
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