IBM 3745 Series, Справочник по техническому обслуживанию оборудования

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the DE status to the host when the CA returns to the neutral state. This DE
status is tagged for that interface by the hardware to clear the busy status pre-
viously presented to the host. The tagged DE status frees the host so that it
can issue the next command.
3. Untagged Asynchronous Status. When operating with tightly coupled
processors, the control program can present asynchronous status to the host
to initiate a command. This occurs when the CA is in the neutral state with no
commands active.
'Request in' is raised to both interfaces, and both interfaces compete for
service. The first channel to poll the CA wins. Acceptance of the status
causes a level 3 interrupt request, and the CA returns to neutral. If the channel
stacks the status, the CA returns to neutral and interrupts the control program.
The untagged asynchronous status is then re-offered to both interfaces by the
CA when the interrupt is serviced by the control program.
Presentation of Status
Although the CA has two channel interfaces, simultaneous operation of both inter-
faces is impossible. Therefore, when the CA is switched to one interface, they
cannot operate simultaneously. The other interface refuses all channel-initiated
sequences. When the CA is in long-term or contingent allegiance on one interface,
the other interface presents the status X'10' (short busy sequence) to the channel:
When 'select out' is trapped by the non-switched interface, it raises 'status in' and
presents X'10'. The host channel responds by dropping 'hold out' or 'select out'
and 'address out', thereby disconnecting the interface. The host does not issue
further commands until a tagged DE status is received. This status is presented
when the active CA interface returns to the neutral state. The CA hardware raises
'request in' and presents the DE status. If the DE status is stacked instead of
accepted, the CA automatically tries to present the status again, independently of
the control program.
Effect of System Reset
System Reset over Interface with Allegiance: When the CA recognizes the
system reset, the CA is completely reset, ending any allegiance condition, and sets
an initial selection interrupt request on level 3. However, if a DE status resulting
from a previous busy status on the opposite interface is still pending, the CA is not
reset.
During system reset, if the opposite channel polls the CA in response to a 'request
in' from some other control unit, the resulting 'select out' tag is bypassed. Similarly,
any channel-initiated initial selection sequence to either interface causes the CA to
switch to that interface, to present the busy status, and to return to the neutral
state.
System Reset over Interface without Allegiance: When a system reset occurs
on the interface without allegiance, any tagged DE status pending is reset. The
rest of the CA is not reset, and no level 3 initial selection interrupt request is made.
Chapter 7. Channel Adapter (CA)
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