Chapter 2
THEORY OF OPERATION
1.0 Introduction
This Chapter provides a detailed theory of operation for the Controlhead circuits. For details of the
trouble shooting refer to the related Section of this manual.
2.0 Controlhead Model for GM140, GM340 and GM640
The controlhead contains the internal speaker, the on/off/volume knob, the microphone connector,
several buttons to operate the radio and several indicator Light Emitting Diodes (LED) to inform the
user about the radio status. To control the LED’s and to communicate with the host radio the control-
head uses the Motorola 68HC11E9 microprocessor.
2.1 Power Supplies
The power supply to the controlhead is taken from the host radio’s FLT A+ voltage via connector
J0801 pin 3 and the regulated +5V via connector J0801 pin 7. The voltage FLT A+ is at supply
voltage level and is used for the LED’s, the back light and to power up the radio via on / off / volume
knob. The stabilized +5 volt is used for the microprocessor and the keypad buttons. The voltage USW
5V derived from the FLT A+ voltage and stabilized by the series combination of R0822, VR0822 is
used to buffer the internal RAM of the microprocessor (U0831). C0822 allows the supply voltage
level to be disconnected for a couple of seconds without losing RAM parameters. Dual diode D0822
prevents radio circuitry from discharging this capacitor. When the supply voltage is applied to the
radio, C0822 is charged via R0822 and D0822. To avoid, that the µP enters the wrong mode when
the radio is switched on while the voltage across C0822 is still too low, the regulated 5V charge
C0822 via diode D0822.
2.2 Power On / Off
The On/Off/Volume knob when pressed switches the radio’s voltage regulators on by connecting line
ON OFF CONTROL to line UNSW 5V via D0821. Additionally, 5 volts at the base of digital transistor
Q0822 informs the controlhead’s microprocessor about the pressed knob. The microprocessor
asserts pin 62 and line CH REQUEST low to hold line ON OFF CONTROL at 5 volts via Q0823 and
D0821. 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. If the radio was switched off, the
radio’s µP will switch it on and vice versa. If the On/Off/Volume knob is pressed while the radio is on,
the software detects a low state on line ON OFF SENSE, the radio is alerted via line ON OFF
CONTROL and sends a data request message. The controlhead µP will inform the radio about the
pressed knob and the radio’s µP will switch the radio off.
2.3 Microprocessor Circuit
The controlhead uses the Motorola 68HC11E9 microprocessor (µP) (U0831) to control the LED’s
and to communicate with the host radio. RAM and ROM are contained within the microprocessor
itself.
The microprocessor generates it’s clock using the oscillator inside the microprocessor along with a 8
MHz ceramic resonator (U0833) and R0920.
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