Epson PX-8 Technical Manual page 115

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