Interrupt Poll And Data Poll - HP 3000 III Series Manual

I/O System
CENTRAL (CTL) DATA BUS
CPU
Data
Poll
lOP BUS
SELECTOR CHANNEL BUS·
'Device
Cont.
r--- -
I
I
I
I DIRECT 1/0 DEVICE
I
LEONTROL':!R
..J
MUX CHANNEL BUS
Figure 7-11. Interrupt Poll and Data Poll
Note
The service request number has no asociation with
the devuce number. It is simply a convenient way
by which a Mul tiplexer Channel can commun ica te
with and assign priorities to its set of Device
Controllers.
For high-speed Device Controllers, the Port Controller ·determines
the first level of priority. Selector Channell has highest pri-
ority and Selector Channel 4 has lowest priority. Although three
Selector Channel ports are available (port 2 cannot be used), two
Selector Channels is the maximum system configuration. The sec-
ond-level determination is a simple preemptive process. The
first device to be given an SIO instruction on a particular chan-
nel will have exclusive use of that channe 1 unt il its I/O p ro-
gram is finished. No further SIO instructions for devices con-
nected to that channel can be honored until that time.
7-21