IBM Series 1 - Page 32

C
c
o
Figure
Poll
Poll
prime
2-6. Poll
bypass
I/O
attachment
card
N
r-
Poll
propagate
MI2
Mll
MI3
'---
mechanism
I/O
I/O
attachment
attachment
card
card
N+I
N+2
,--
Poll Poll
Poll
propagate
Poll
propagate
MI2
MIl MI2 MIl
Poll
Poll
prime
prime
M13
M13
-
'---
foIl f£su~~at~. This ou·tbound tag is raised by an I/O
device when it receives a poll that it has not captured.
Poll Retu~. This is an inbound tag sent by an I/O device
to signal the channel that a poll capture, for an interrupt
pallor for a non-burst cycle steal service has taken place.
It is not used to signify that a burst transfer is required.
Burst Ret.!!!:!!. 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 a burst transfer is required. Once
activated. the next leading edge of a service gate tag
signals the beginning of the burst transfer. The entire
channel is now dedicated to the I/O device. Service
gate/service gate return "hand shaking". 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 control of the program. 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. Note that DCB fetching is always done in
non-burst .ode. Burst mode is not precluded for IPL
transfers.
Processor I/O Channel 2-17