REV.-A
2.9.5
Barcode
Input
(BRDT)
The barcode
input
(BRDT) terminal is
connected
to
the
AN3
terminal
of
the
A-D
converter,
through
voltage
limiter
and noise
limiter
circuits
similar
to
the
ANIN signal line described above,
and
pins
1 and 2
of
the
NAND
gate 3D. The
NAND
gate feeds
the
barcode data
input
signal
to
the
read circuit,
which
is discussed in detail in
the
next
section,
"Barcode
Interface". The
AN3
channel
provides a route
to
a special
check
function
for
any unacceptable
deviation
from
the
nominal
vol-
tage levels,
+5V
and
the
ground
level,
of
the
barcode data signal.
Barcode
Interface
There are
may
barcode
systems
for
each
of
which
a
different
barcode data read
progam
is re-
quired. In addition, there are numerous
models
of
barcode readers
whose
hardware
characteris-
tics
(which
require
different
barcode
color
pattern
specifications,
scanning
angles,
and/or
reading
heights, etc.). This
computer
incorporates
an
interface
for
TTL-compatible,
hand-held barcode
readers
which
are discussed in detail in
the
following:
This interface has a
+5V
line
terminal
that
can be used
to
supply
the
operating
power
to
the
con-
nected barcode reader. The
supply
is
controlled
by
the
barcode reader
power
on/off
(SWBCD)
signal fed
from
port
12
of
the
gate array
GAH40M
which
can be
directly
controlled
by
bit
B
of
main
CPU
address
00.
The barcode data signal line is supplied
to
the
AN3
terminal
of
the
A-D
converter,
through
a
vol-
tage
limiter
and noise
eliminator
circuits, similarl
to
the
ANIN signal line, and
port
14
of
GAH40M
(BCD)
after
being inverted
by
the
IC
3D
as
described above. There
the
signal is examined
whether
it
is active (ON
or
MARK)
or
inactive (OFF
or
SPACE), and each active (MARK)
duration
(i.e.,
the
pulse
width)
is measured under
the
control
of
the
main CPU. Before proceeding
to
the
discussions
on
the
barcode interface
circuit
operations, basic
functional
theories
of
a barcode reader and a
sample
waveform
which
it
generates
from
a given barcode
pattern
are discussed here.
Barcode
Reader
Barcode sensor
+5O----~---------+--------~LE~D~--~
BRDT
Digitizer
circuit
Amplifier
t----~
Photo
transistor
I
Barcode marks printed
on label sheet
Lens ,
':::~.:::.:::--
--::=:-.:::-
1
'-
T
--
Reflected
light
Fig.
2-93
Barcode
Reader
Funcational
Block
Diagram
2-91
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