Chapter 3 Theory Of Operation; W3 Control Head; Sb9600 Serial Bus Interface; Display Circuit - Motorola ASTRO W3 Service Manual

Chapter 3 Theory of Operation

This chapter describes the operational theory of the control heads for the W3, W4, W5, W7, and W9
radio models. It includes vehicular interface port (VIP) information.
3.1

W3 Control Head

The W3 Hand-Held Control Head (HHCH) contains the U3 microcomputer, which runs on a 4 MHz
clock and performs the following basic functions:
• SB9600 data encode and decode for communication to host radio.
• Liquid-crystal display (LCD) drive comprising two rows of 14 dot-matrix characters plus
10 annunciators.
• Scans a 22-key keypad.
• Scans three side keys and three top keys.
• Determines what type of radio is connected from the bus data and selects either portable or
mobile/vehicle adaptor (VA) modes.
• Scans the PTT switch.
• Provides display drive and programmable adjustment for display contrast.
• Provides display and keypad backlight control.
• Activates the red PTT LED and yellow status LED as required.
• Provides remote keying of the microphone via the SB9600 serial bus as required.
• Detects off-hook/on-hook conditions in mobile/VA mode.
• Provides SPI serial communications to U8 for serial-to-parallel output expansion, U12 for
parallel-to-serial input compression, U14 for analog-to-serial input compression, and option
connector J2.
3.1.1

SB9600 Serial Bus Interface

The serial interface circuit, comprising transistors Q1, Q2, and Q8, and comparator U2, provides an
interface between the microphone/message and a host radio for two-way SB9600 serial bus
communications. This is necessary to control the radio from the HHCH and have radio status
messages displayed on the HHCH. The interface circuit performs the following functions:
• Buffering of the input and output data by Q1 and U2 to improve isolation and drive capability.
• Conversion of balanced to unbalanced SB9600 data input to SB96 bus interface by Q1, U2.
• Bidirectional busy control through U3-20, -21, and -41 to J 1-3. U320 and -21 are paralleled to
provide increased drive capability.
3.1.2

Display Circuit

The display is a liquid-crystal display (LCD) and comprises two rows of 14 dot-matrix characters and
10 annunciators. The display circuit consists of sections involving character generation, backplane
drive, negative voltage generation, temperature compensation, and contrast control. A 200 kHz clock
is derived from U4-25, which also supplies U14.