IBM Series 1 - Page 31

Poll and Pol! Prim~ '!:gg.§. The poll tag is a serially
propagated tag generated by the channel to resolve
contention between multiple devices requesting interruptions
on the same level and cycle steal requests. Each I/O device
receives the poll tag andredrives it to the next device on
the I/O channel via its poll propagate output line.
The poll and poll prime tags are always recognizable by
an I/O device that is capable of presenting interrupts or
cycle stealing. An I/O device recognizes a poll as the
leading edge of the logical "AND" of poll and poll prime.
The device does a logical compare between the poll ID bits
on the interface and the ID bits in his prepare register.
An equal compare of the poll ID Bits "Anded" with poll and
poll prime is called poll capture. Once the poll is
captured, an I/O device can then respond with a poll return
or burst return. If the poll captuI:'e does not occur, the
poll tag is propagated to the next device on the 1/0
channel.
Because of the serial nature of the poll mechanism, the
relative physical position of I/O attachments on the channel
is a major determinant of the priority for serv1c1ng
contending cycle steal requests and for servicing contending
interrupt requests on the same interrupt level. Those
attachments located in positions nearer to the processor are
the first elements on the serial poll chain and are
therefore of higher priority.
To facilitate the removal of an I/O attachment card
from the I/O channel without interrupting t.he poll
propagation, a bypass mechanism is provided. Reference
Figure 2-6. The poll prime tag, received by I/O attachment
card N+2, is the salle line that I/O attachment card N+1
receives as its poll tag. Hence, if I/O attachment card N
is removed, the poll tag input to attachment card N+1 and
the poll prime input to attachment card N+2 will appear as a
logical 1. The poll prime input to attachment card N+1 will
then follow the poll propagate output of attachment card
N-1.
If an I/O device does not present interruptions or
cycle steal requests, the poll and poll prime tags are
received, logically "ANDed", and then redriven to the poll
propagate output. Cards that use an I/O slot on the
channel, but are never logically connected to the channel as
a device attachment, must also implement this logical
"ANDUing and redrive capability.
1. The poll bypass function applies on and between 110
units.
2. The cable slots or repower card are not considered to
be an I/O attachment for purposes of the poll bypass
mechanism, e.g., it is not permissible to leave an
empty I/O slot on each side of the repover card.
2-16 GA34-0033
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