821818226
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
componenu,
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,
1/0, 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
!ide
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 (350mV worst case).
Control
aatlng
OlEN,
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
OlEN input.
The
OlEN 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
is
a device
that
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
01.
()O------p_-+-.,
08.
DO.
<>---t--G--+--'
01,
o---t--t>--+---,
08,
DO,o---t--G--+--'
01,
o---t--t>--+---,
08,
DO,o---t--G--+--'
01,
o---t---t~-+---..,
DB,
DO,
C>---t--c.I--+----'
'-----t
.....
----o
cs
OlEN
01.
DO.
01,
DO,
01,
DO,
01,
DO,
OlEN
CS
0 0
01
. DB
,
0
DB·
00
t-+
t+
, ,
} HIGH IMPEDANCE
Figure 1. 821818228 Logic Diagrams
8-39
AFN-00733A-Q2
