Motorola GM1200E, Service Manual

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Theory of Operation
Introduction/Theory of Operation 2.1-5
When the host radio needs to communicate to the control head
µ
P it brings the control head select
line (CNTL HD CE) to a logic ‘0’ and then sends the proper data and clock signals. After the data has
been sent the control head select line is returned to a logic ‘1’. When the control head
µ
P wants to
communicate to the host radio the
µ
P brings request line CNTL HD REQ to a logic ‘0’ by switching
on transistor Q0931 via
µ
P pin 11. The host radio then starts communication by activating the
control head select line (CNTL HD CE), sending the clock signal and sending data via SPI MOSI or
receiving data via SPI MISO and buffer U0931-1.
3.7 Keypad Keys
The control head keypad is a 26 - key keypad. All keys are configured as 6 analogue lines (AN 0 - 5)
to the control head
µ
P. Lines AN 0 - 3 each control four keys, lines AN 4, 5 each control five keys.
The voltage on the analogue lines varies between 0V and +5V depending on which key has been
pressed. If a button is pressed, it will connect one of the 6 lines AN 0 - 5 to a resistive voltage divider
R0807 - R0811 connected to +5V. The voltages of the lines are A/D converted inside the
µ
P (ports
PE 0 - 5) and specify the pressed button.
3.8 Status LED and Back Light Circuit
All the indicator LEDs (D0881 - D0884) are driven by current sources Q0881 - Q0883. To change
the LED status the host radio sends a data message via SERIAL PERIPHERAL INTERFACE (SPI)
to the control head
µ
P. The control head
µ
P determines the LED status from the received message
and switches the LEDs on or off via pins 5, 6, 7. The LED status is stored in the
µ
P’s memory. The
LED current is determined by the resistor at the emitter of the respective current source transistor.
The backlight for the LCD and the keypad is controlled by the host radio the same way as the
indicator LEDs using
µ
P pins 8, 9, 10. The keypad backlight current is drawn from the FLT A+ source
and controlled by transistor Q0851. The current flowing through the LEDs cause a proportional
voltage drop across the parallel resistors R0861, R0862. This voltage drop is amplified by the op-
amp U0831-2. U0831-2 and Q0852 form a differential amplifier. The voltage difference between the
base of Q0852 and the output of U0831-2 determines the current from the base of the LED control
transistor Q0851 and in turn the brightness of the LEDs. The
µ
P can switch the LEDs on and off by
a logic high or low level at the port connected to the base of Q0852. If the base of Q0852 is at
ground level, Q0852 is switched off and no current flows through Q0851 and the LEDs. If the
µ
P port
changes to +5V a current flows through Q0852 and in turn through Q0851 causing the LEDs to turn
on and a rising voltage drop across R0861, R0862. The rising voltage causes the output of the op-
amp to rise and to reduce the base to emitter voltage of Q0852. This decreases the current of
Q0852 until the loop has settled. The backlight for the LCD uses a similar circuit. By using two
µ
P
ports (pin 8, 9) and different weighting resistors R0837 and R0838 the base of Q0832 can be set to
four different voltage levels. This allows to switch the LEDs off or to select among three levels of
brightness.
3.9 Liquid Crystal Display (LCD)
The LCD module U0902 consists of the display and the display driver. The display is a single layer
super twist nematic (STN) LCD display. It has a dot matrix of 24 x120 dots for displaying graphics
and alpha - numerical information, a line with 19 pre - defined icons below the dot matrix and line
with 11 bars below the icon line. Six of the bars can be used to display the status of the keys located
below.