IBM 1 Series - Page 25

2-14 GA34-0033
If an I/O device does not present interrupt 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 ANDing and redrive
capability.
Notes:
1. The poll bypass function applies on and between the processor and
I/O expansion units.
2. The cable slots and repower card are not considered to be an I/O
attachment for purposes of the poll bypass mechanism (it is not
permissible to leave an empty I/O slot on each side of the repower
card).
Poll Propagate. This outbound tag is sent by an I/O device when it
receives a poll that it has not captured.
Poll Return. This is an inbound tag sent by an I/O device to signal the
channel that a poll capture for an interrupt poll or for a non-burst
cycle-steal service poll has taken place. It is not used to signify that a
burst transfer is required.
Burst Return. This is an inbound tag sent by an I/O device to signal the
channel that a poll capture (cycle-steal only) has taken place and that a
burst transfer is required. Once ‘burst return’ is activated, the next leading
edge of a ‘service gate’ tag signals the beginning of the burst transfer.
During the burst operation, all other channel-attached devices are
precluded from using the channel. ‘Service gate’/‘service gate return’
‘handshaking,’ between the channel and the I/O device, continues until
the ‘burst return’ tag is dropped. An I/O device deactivates the ‘burst
return’ tag at the rise of the ‘service gate’ tag for the last transfer. Under
program control, burst mode is used only if so specified in the DCB
control word. A device capable of supporting burst mode must also be
capable of non-burst operations. (DCB fetching is always done in
non-burst mode.) Burst mode is not precluded for IPL transfers.