Electronic On/Off; Mechanical On/Off; Ignition - Motorola GM950E Service Manual

The voltage SW B+ is monitored by the µP through the voltage divider R0641/R0642. Diode VR0641
limits the divided voltage to 5.1V to protect the µP.
Diode D5601 (UHF) / D3601 (VHF) located on the PA section acts as protection against transients
and wrong polarity of the supply voltage.
2.3

Electronic On/Off

The radio has circuitry which allows radio software and/or external triggers to turn the radio on or off
without direct user action. For example, automatic turn on when ignition is sensed and off when
ignition is off.
Q0611 is used to provide SW B+ to the various radio circuits. Q0611 acts as an electronic on/off
switch controlled by Q0612. The switch is on when the collector of Q0612 is low. When the radio is
off Q0612 is cutoff and the voltage at Q0611-base is at A+. This effectively prevents current flow
through Q0611 from emitter to collector. When the radio is turned on the voltage at the base of
Q0612 is high (about 0.6V) and Q0612 switches on (saturation) and pulls down the voltage at
Q0611-base. With Transistor Q0611 now enabled current flows through the device. This path has a
very low impedance (less than 1 ohm) from emitter to collector. This effectively provides the same
voltage level at SWB+ as at A+.
The electronic on/off circuitry can be enabled by the microprocessor (through AFIC port GCB1, line
B+ CONTROL), the mechanical On/Off button on the control head (line ON OFF CONTROL), or the
ignition sense circuitry (line IGNITION CONTROL). If any of the 3 paths cause a low at the collector
of Q0612, the electronic ON is engaged.
2.4

Mechanical On/Off

This refers to the on/off button, located on the control head, and which turns the radio on and off. If
the radio is turned off and the on/off button is pressed, line ON OFF CONTROL goes high and
switches the radio on as long as the button is pressed. The microprocessor is alerted through line
ANALOG 3 which is pulled to low by Q0925 (Control Head Model A3) while the on/off button is
pressed. If the software detects a low state it asserts B+ CONTROL via AFIC pin 39 low which
keeps Q0612 and Q0611, and in turn the radio switched on.
If the on/off button is pressed and held while the radio is on, the software detects the line ANALOG
3 changing to low and switches the radio off by setting B+ CONTROL to low.
2.5

Ignition

Ignition sense is used to prevent the radio from draining the vehicle's battery because the engine is
not running.
When the IGNITION input (J0400- 10) goes above 6 volts Q0421 and Q0612 turn on. This turns on
SW B+ by turning on Q0611 via line IGNITION CONTROL and Q0612 and the microprocessor
starts execution. The software reads the line IGNITION SENSE, determines from the level that the
IGNITION input is active and sets the B+ CONTROL output of the AFIC pin 39 to high to latch on
SW B+.
Theory of Operation
Controller
6.3-3