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SCHOTTKY BIPOLAR
8212
BI-DIRECTIONAL BUS DRIVER
III. Bi-Directional Bus 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
D81
on the first
8212
and to D82 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.
DATA---~
BUS
DATA
BUS
CONTROL
(o=
L -
R)
(I
= R-
L)
STB
8212
GND
1----'---.........
OAT
A
BUS
STB
8212
GND
TO
PRIORITY
CKT
_-...-
(ACTIVE LOW)
OR .
TO
CPU
INTERRUPT INPUT
DATA
BUS
STB
INTERRUPTING INPUT PORT
INPUT
STROBE
SYSTEM
INPUT
SYSTEM
RESET
PORT {
SELECTION
(DS1.DS2)
--------'
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 instruc-
tion, 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. (081 could be used to multiplex a
variety
of
interrupt instruction ports onto a com-
mon bus).
INTERRUPT INSTRUCTION PORT
DATA
BUS
STB
RESTART
INSTRUCTION
(RST
0-
RST
7)
(DSI) PORT SELECTION - ........
~
INTERRUPT ACKNOWLEDGE
-...--_
..........
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