Bus Transceiver; Vacuum Fluorescent Voltage Converter; Vehicle Interface Ports; Power Supply - Motorola ASTRO W3 Service Manual

3-10

3.3.3.3 Bus Transceiver

The serial bus transceiver is on SIOIC U4. The timer on SIOIC U4, pins 10, 16, and 17, connects to
the external bus. Pins 11, 13, 19, and 20 connect to MPO 2. These pins act as a serial-
communication buffer.

3.3.3.4 Vacuum Fluorescent Voltage Converter

Voltage for the vacuum fluorescent (VF) display filaments is generated by a fixed-frequency,
variable-duty-cycle-driven, flyback voltage converter. Dual Comparator U3B, with resistors R56
through R60, form a triangle-wave generator that has an output swing between 1.5 and 3.2 volts at a
frequency of 160 kHz. The output of this triangle goes to U3A-3, where it is compared to a feedback
signal on U3A-2. The d.c. feedback signal comes from the voltage converter output through diode
CR51 and is regulated by VR50 to 43 volts. The U3A output voltage increases or decreases as its
input voltage rises or falls. Consequently, the duty cycle seen at the gate of MOSFET transistor Q50
varies inversely with the triangle output voltage. The output of Q50, varying at 160 kHz, is applied to
the primary of transformer T60, pins 1 and 3. The secondary of T60, pins 4 and 6, supplies 3.5 Vac
for the VF display (U102) filaments.

3.3.3.5 Vehicle Interface Ports

The control-head connector, P103, provides connections for three vehicle interface port (VIP) inputs
and three VIP outputs.
The three VIP inputs enter the control head at P103, pins 4, 3, and 37. Zener diodes YR10, YR9, and
YR8 limit the input voltages to 15 volts. VIP input 2 can be strapped through jumper JU11 to provide
various control functions connected to P103, pins 41, 43, and 44. The three inputs are connected to
SIOIC U4 at pins 25, 22, and 35. The input signals are buffered in U4 and are output on pins 24, 21,
and 36. From U4, the signals are fed through 10K resistors R44, R14, and R15 to the MPU, U2. The
MPU directs the signals as required by programming.
Programmable switches on the control head are connected through J101, pins 11, 10, 9, 8, and 7
(KRl through KRS, respectively) to CPU U2, pins 49, 44, 46, 48, and 50. The other contacts of the
switches are connected through J101, pins 14, 15, 16, 17, and 19 (KSl through KSS), to CMOS shift
register U6, pins 4, 5, 6, 7, and 14.
Shift register U6 outputs, pins 13, 12, and 11, go to Darlington transistors Q71, Q72, and Q73. These
three transistors drive the VIP outputs (up to 300 mA each) through control-head connector PI03,
pins 2, 1, and 34. Primarily, these transistors control external relays. VIP outputs 1 and 2 are
connected to jumpers JU12 and JU13 to provide control functions connected to P103, pins 28 and
29. VIP output 1 can also be connected through JU22 to MIC-PTT, pin 24.

3.3.3.6 Power Supply

The +5-volt power supply U46 is a three-terminal regulator IC that regulates the 12-volt SWB+ down
to +5 volts for the digital logic circuits. The regulated +5 volts enters the control-head PC board
through connector J101, pin 4.

3.3.3.7 Ignition Sense Circuits

SIOIC U4 senses the vehicle ignition's state through its pin 28, disabling transmit when the ignition is
off. For negative-ground systems, the orange lead is typically connected to +l2 volts. For more
information, see the cable kit section in the Detailed Service Manual 68P80102W61.
NOTE: Q40 supplies an inverting path for positive-ground ignition service.
September 26, 2003
Theory of Operation: W9 Control Head
6881096C77-O