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III. BI-Dlrectlonal
BUB
Driver
A pair
of
8212's wired (back-to-back) can
be
used as a
symmetrical drive, bi-directional bus driver. The devices
are
controlled by the data bus
input
control which is
connected to
DS1
on the first 8212 and
to
DS2
on the
second. One device is active, and acting as a straight
through buffer the other
is
in
3-state mode. This is a very
useful
circuit
in small system design.
IV.
Interrupting Input Port
This use
of
an 8212 is that
of
a system
input
port that
accepts a strobe from the system input source, which in
turn clears the service request flip-flop and interrupts the
processor. The processor then goes through a service
routine, identifies the port, and causes the device
selection
logic
to
go
true
-enabling
the system input data
onto the data bus.
V.
Interrupt Instruction Port
The 8212 can be used to gate the interrupt instruction,
normally RESTART instructions, onto the data bus. The
device is enabled from the interrupt acknowledge signal
from the microprocessor and from a port selection signal.
This signal is normally tied to ground.
(DS1
could
be
used
to multiplex a variety
of
interrupt instruction ports
onto
a
common bus).
8212
8-27
BI-DIRECTIONAL BUS DRIVER
DATA
BUS
DATA
BUS
CONTROL
(0=
L -
RJ
(I
= R -
LJ
I
~
-
"-
:.
v
--c
J.
STB
8212
CLR
Y
G~D
L--
STB
,.l
DATA
BUS
~
8212
L
CLR
----..J
f---Y
GND
INTERRUPTING INPUT PORT
INPUT
STROBE
SYSTEM
INPUT
SYSTEM
RESET
STB
"
DATA
BUS
PORT {
SELECTION
(DS1·DS2J
-------'
t-----
T~c~~0~R~6~JCKT
INTERRUPT INSTRUCTION PORT
RESTART
INSTRUCTION
(RST 0 -
RST
7J
(DSIJ
PORT SELECTION
OR
STB
INTERRUPT ACKNOWLEDGE
_
........
___
--l
TO
CPU
INTERRUPT INPUT
DATA
BUS
AFN·OO731A·04