Controlhead Model For Gm398; Power Supplies; Voltage Regulator Circuit; Power On / Off - Motorola GM328 Service Manual

Controlhead Model for GM398

4.0 Controlhead Model for GM398
The controlhead contains the On/Off/Volume knob, the microphone connector, several buttons to
operate the radio, several indicator Light Emitting Diodes (LED) to inform the user about the radio
status, and a Liquid Crystal Display (LCD) with 21 pre - defined symbols and a 32*96 dot matrix for
graphical or alpha - numerical information e.g. channel number, select code, call address name. To
control the LEDs and the LCD, and to communicate with the host radio the controlhead uses the
Motorola 68HC11K4 microprocessor.
4.1

Power Supplies

The power supply to the controlhead is taken from the host radio's FLT A+ voltage via connector
J0801 pin 3. The voltage FLT A+ is at battery level and is used for the LEDs, the back light, to power
up the radio via On/Off/Volume knob and to supply the voltage regulator circuitry. The regulator
circuitry provides the stabilized +5 volts which is used for the microprocessor circuitry, the display,
the display driver and the keypad buttons. The regulated +5V taken from the host radio via
connector J0801 pin 7 (line 5V SOURCE) is only used to switch on or off the voltage regulator in the
control-head.
4.2

Voltage Regulator Circuit

Voltage regulator U0861provides 5V for the controlhead. The supply voltage FLT A+ for the voltage
regulator is fed via parallel resistors R0861/2 and dual diode D0861 to pin 8 of U0861. The +5 volt
output is switched on and off by the host radios's 5 volt source via line 5V SOURCE and control
transistor Q0866. When the host radio is switched off the voltage on line +5V SOURCE is at ground
level and switches off transistor Q0866. Pull up resistor R0863 pulls input SHUTDOWN (pin 3) of the
voltage regulator U0861 to FLT A+ level and switches off the output of U0861 (pin 1). When the host
radio is switched on the voltage on line 5V SOURCE of about +5 volts switches on transistor Q0866
which in turn pulls input SHUTDOWN (pin 3) to ground and switches on the output of U0861. Input
and output capacitors (C0861 / C0862 and C0864 / C0865) are used to reduce high frequency noise
and provide proper operation during battery transients. Diode D0861 prevents discharge of C0862
by negative spikes on the FLT A+ voltage. This regulator provides a reset output (pin 5) that goes to
0 volts if the regulator output goes out of regulation. This is used to reset the microprocessor
(U0871) and the display driver (J0821-5) to prevent improper operation.
The voltage USW 5V derived from voltage FLT A+ is stabilized using resistor R0855 and diode
VR0855 This voltage is used to buffer the microprocessor's internal RAM. C0856 allows the battery
voltage to be disconnected for a couple of seconds without losing RAM parameters. Diode D0855
prevents radio circuitry from discharging this capacitor. The +5V at the second anode of D0855
speeds up charging of C0856, when the host radio is turned on by a high level at the ignition input
while the supply voltage is applied to the radio. This prevents the microprocessor from accidently
entering bootstrap mode.
4.3

Power On / Off

The On/Off/Volume knob when pressed switches the radio's and the controlhead's voltage
regulators on by connecting line ON OFF CONTROL to line UNSW 5V via D0852. Additionally, 5
volts at the base of digital transistor Q0853 informs the controlhead's microprocessor about the
pressed knob. The microprocessor asserts pin 8 and line CH REQUEST low to hold line ON OFF
CONTROL at 5 volts via Q0852 and D0852. The high line ON OFF CONTROL also informs the host
radio, that the controlhead's microprocessor wants to send data via SBEP bus. When the radio
returns a data request message, the microprocessor will inform the radio about the pressed knob.
2-9