Symbol LS 1220 Product Reference Manual page 21

LS 1220 Product Reference Guide
Motor Driver
The Motor Driver is an electromagnetic and electronic circuit that provides feedback control
of the mylar motor scan element. The circuit regulates the scan amplitude of the motor/
mirror assembly. The scan frequency is determined by the resonance characteristics of the
mechanical design. The motor fail detector is a laser safety circuit that monitors the motor
behavior, and turns off the VLD if the motor fails to operate. The SOS (Start Of Scan) signal
transitions from high to low and low to high, corresponding to the edges of the scan line.
The signal frames the data received by a complete scan line.
Control Circuitry
Interface Control Circuitry controls operation of the scanner, motor, and laser, depending
on the states of the input signals from the host device.
Photodiode
The Photodiode is a transducer that converts incident light energy into an electrical current.
It is the "eye" of the scan engine. When the laser beam passes over a barcode, the black
bars absorb the light and the white spaces reflect the light. Collection optics focus the
received reflected light onto the photodiode. The photodiode produces a photocurrent
proportional to the received optical signal.
Analog Signal Processor
The Analog Signal Processor is a transimpedance preamplifier which converts the
photocurrent into a voltage and provides amplification. Additional amplifier stages provide
signal gain and bandpass filtering. The AGC (Automatic Gain Control) circuit is a feedback
loop that monitors the received signal voltage level and varies the voltage gain to maintain
a constant amplitude at the output. The output analog signal is then input into the digitizer.
Digitizer
The Digitizer is an edge detection circuit that takes the amplified and filtered analog signal
and converts it into a digital representation of the scanned barcode. The output of the
digitizer is called the DBP (Digitized Bar Pattern). The widths of the DBP elements are
proportional to the printed bars and spaces of the barcode. The DBP signal is sent to the
decoder board or host computer to decode the data.
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