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8216/8226
FUNCTIONAL
DESCRIPTION
Microprocessors
like
the
8080
are
MOS
devices
and
are
generally
capable
of driving
a single
TTL
load.
The
same
is
true for
MOS
memory
devices.
While
this
type
of
drive
is
sufficient
in
small
systems with few
components,
quite often
it
is
necessary
to buffer
the microprocessor
and memories
when
adding
components
or
expanding
to
a
multi-board
system.
The 8216/8226
is
a
four
bit
bi-directional
bus
driver
specif-
ically
designed
to buffer
microcomputer
system
components.
Bidirectional Driver
Each
buffered
line
of the four
bit
driver consists
of
two
separate buffers that are
tri-state in
nature
to
achieve
direct
bus
interface
and
bi-directional capability.
On
one
side
of
the
driver
the
output
of
one
buffer
and
the input
of another
are
tied
together (DB),
this side
is
used
to interface to the
system
side
components
such
as
memories,
I/O,
etc.,
be-
cause
its
interface
is
direct
TTL
compatible and
it
has high
drive
(50mA).
On
the other
side
of the driver the inputs
and outputs
are
separated
to
provide
maximum
flexibility.
Of
course,
they can be
tied
together so
that the driver
can
be used
to
buffer
a
true bi-directional
bus such
as
the
8080
Data
Bus.
The
DO
outputs on
this
side
of the driver
have
a
special
high voltage
output
drive capability
(3.65V)
so that
direct interface to
the
8080
and
8008
CPUs
is
achieved with
an
adequate
amount
of noise
immunity
(350m
V
worst
case)
Control
Gating DIEN,
CS
The CS
input
is
actually
a
device
select.
When
it
is
"high"
the
output
drivers are
all
forced
to their
high-impedance
state.
When
it
is
at
"zero"
the device
is
selected (enabled)
and
the direction
of
the data flow
is
determined by
the
DIEN
input.
The
DIEN
input controls the
direction of
data flow
(see
Figure
1)
for
complete
truth
table.
This
direction control
is
accomplished by
forcing
one
of the
pair
of buffers
into
its
high
impedance
state
and
allowing the other
to
transmit
its
data.
A
simple
two
gate
circuit
is
used
for
this
function.
The 8216/8226
isadevicethat
will
reduce
component
count
in
microcomputer
systems and
at
the
same
time enhance
noise
immunity
to assure
reliable,
high
performance
op-
eration.
(a)
8216
(b)
8226
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IT
11
DIEN
CS
Dl
DB
1
DB
•
DO
1
hHIGH
IMPEDANCE
1
1
Figure
1.
8216/8226 Logic
Diagrams
7-46
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