User`s guide

The XMI
2.9.1.3 Transaction Timeout
The XMI protocol specifies that a timeout of 16 milliseconds be used
by commanders to detect transaction failure. Responders ensure that
transactions do not exceed these timeout values.
• Response Timeout—An XMI Read, Interlock Read, or IDENT
transaction is considered to have failed if a commander does not
receive all read responses before the timeout cycle value expires. This
does not imply that a responder has "died" since XMI receivers ignore
cycles with bad parity and response timeouts can occur as a result of
ignored cycles.
• Retry Timeout—An XMI commander needs to reissue an XMI
transaction if it receives a NO ACK or a Locked Response. If the
commander has not successfully completed the transaction within the
timeout period, the transaction has failed.
2.9.1.4 Sequence Error
Many transactions require that XMI cycles occur in a certain sequence.
When the cycles occur out of sequence, the transaction is in error.
Read, Interlock Read, and IDENT transactions use sequence IDs
embedded in the read data responses (GRDn, CRDn, RER—the sequence
ID for RER is implicitly 0). The required order for read responses is 0
(GRD0) for longwords (including IDENT), 0 (GRD0) for a quadword, 0...1
(GRD0, GRD1) for an octaword, and 0...3 (GRD0, GRD1, GRD2, GRD3) for
hexword length transactions. For example, if the commander detects data
returned out of sequence (such as GRD0, GRD2, GRD3), then it NO ACKs
the out-of-order read response (GRD2) and the subsequent read response
(GRD3) for that transaction.
Correct sequencing of write transactions is determined by the location
of the data cycles relative to the write command cycle rather than using
sequence IDs, which are used with reads. The write command cycle and
associated write data cycles must occur in contiguous timeslots. If a
responder detects missing data cycles in a write transaction, the incorrect
cycle (and subsequent data cycles) are NO ACKed. Figure 2–34 shows
examples of failing hexword write transactions. In both examples there
should be data where XXXX appears.
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