Service manual
4023
CIRCUIT
THEORY
P2
i.1
PATTE~N
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I
SHORT
ALL
The
4023
IEEE
Interface
Circuit Theory
All
the
signals on
the
interface
are
controlled
by
the
I/O device U4D. Eight parallel bi-directional data
lines (PBO-PB7) are used
as
the
parallel
data
bus
for
the
interface.
DAV
(Data Valid) pin 6
of
P2 is
a
"high"
active
output
from
the
transmitting
device. Valid 8
bit
codes are
transmitted
to
the
receiv-
ing device
when
DAV
is
"high".
DAC (Data
Accepted)
pin 8
of
P2 is an
output
that
is
"low"
when
data is being accepted because
U4D
pin
10
(PA2) is
"high".
RFD
(Ready
for
Data) pin 7
of
P2 goes
"low",
indicating
the
printer is ready
to
receive data
when
data is
not
being accepted (DAC is
"high"),
and
U4D
pin 7 (PA 1) is
"low".
ATN pin
11
of
P2 is an
input.
The
transmitting
device brings
this
line
"low"
before
the
data
is
transmitted.
EOI
(End or
Identify)
pin 5
of
P2 is an
input.
The
transmitting
device brings
this
line
"low"
when
the
last
byte
of
a message is being
transmitted.
IFC (Interface
Clear) pin 9
of
P2 is an
input.
An
external reset signal applied
to
the
IFC initializes all
the
printer
pro-
cessor logic.
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