Epson PX-8 Technical Manual page 116

REV.-A
As shown in Fig. 2-93, the reader has a light source and a reflected light sensor which connects
the optical barcode pattern (a series of variations of contrast, generated by the black stripes and
exposed white sheet) to a series of electrical pulses. Many barcode readers are of a hand-held
type so that each scan inevitably causes a variation in the output pulse signal (BRDT) due to differ-
ence of scanning speed as shown in Fig. 2-94.
Scanning speed 2
Fig. 2-94 Barcode Data Signal Variation Due To Scanning Speed Difference
If the pattern were read at almost the same scanning speed (i.e., at a constant barcode movement
speed), the ratio of the corresponding marks and spaces would remain the same. Thus, the pat-
tern could be correctly read in principle by supposedly triggering a time measurement mechanism
with each pulse, measuring the time between the pulses, and processing the pulse intervals based
on a reference timing obtained from the measurment. This sequence of operations are actually
accomplished by software as discussed below.
Barcode Data Processing
Read barcode pattern data are detected and processed under the control of the main CPU. A trig-
ger pulse, which is generated by the leading and/or trailing edge of each arriving barcode data
pulse and a free-running counter (an endless counter which repeats counting from 0000 (H) to
FFFF (H) are used to detect the pulses; a data pulse is detected by comparing the counter value at
the time a trigger pulse is generated and at the time the next trigger pulse is generated.
Fig. 2-96 illustrates the operation.
2-92