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The MESSAGE OUT phase is used by the
ACB-550~
to obtain a message
from the Initiator.
The ACB-5500 invokes this phase only in
response to the ATTENTION condition from the Initiator.
In
response to the ATTENTION condition, the ACB-5500 asserts C/O and
MSG and deasserts the I/O signal during the REQ/ACK handshake(s)
of this phase.
The Target handshakes byte(s) in this phase until
the ATTN signal goes false.
4.1.2.6
SIGNAL RESTRICTIONS BETWEEN PHASES
When the BUS is between phases, the following restrictions apply
to the bus signals:
o
The BSY, SEL, REQ and ACK signals may not change.
o
The C/O, I/O, MSG and DATA signals may change.
o
The ATTN and RST signals may change as defined under the
descriptions for the ATTENTION and RESET conditions.
4.1.3
BUS CONDITIONS
The bus has two asynchronous conditions:
The ATTENTION condition
and the RESET condition.
These conditions cause certain BUS
DEVICE actions and can alter the bus phase sequence.
4.1.3.1
ATTENTION CONDITION
ATTENTION allows the Initiator to signal the
ACB-55~0
of a wait-
ing message.
The
ACB-55~0
may access the message by invoking a
MESSAGE OUT phase.
The Initiator creates the ATTENTION condition by asserting ATTN
at any time except during the
ARBITRATION and BUS FREE phase.
The
ACB-55~~
responds when ready with the MESSAGE OUT phase.
The
Initiator
keeps ATTN asserted if more than one byte is to be
transferred.
The Initiator can only deassert the ATTN signal during the RESET
condition, during a BUS FREE phase, or while the REQ signal is
asserted and before the ACK signal is asserted during the last
REQ/ACK handshake of a MESSAGE OUT phase.
While
the ATTN
is asserted,
the Initiator must continue to
respond to the Target command or data transfers since the Target
may not process the ATTN immediately.
4-8
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