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4.2 THEORY OF OPERATION
Microcontroller
The FREESCALE M9S12E64 runs at 24.576MHZ and performs all necessary functions of the Front
Panel assembly: communicates to the CPU via the serial asynchronous port SP-6, scans the keypads
and displays messages on the LCD display, controls the LCD backlight, generates the BELL signal,
monitors the AUX switch and the manual RESET switch.
Keyboards
The keyboard interface is comprised of a 4x4 keyboard (sixteen keys for hexadecimal alphanumeric
entry) and a 3x4 keyboard (twelve keys for cursor control and symbol entry).
All of the lines connected to these two keyboards go directly to the M9S12E64 microcontroller.
P2 and P3 connectors
P2: This 16-positions connector is the interface from the FPA’s PCB assembly to the LCD display. It
carries the data and command lines from microcontroller to the LCD unit, the intensity control from the
contrast knob and the backlighting control.
P3: This 40-positions connector is the interface to the Serial motherboard in order to connect the
communications lines for the SP-6 port and SP-4 port and the control lines ACTIVE LED and RESET
coming from the CPU.
C50S/ C50J Connectors
The C50S is a 9-positions D-sub female connector and the C50J is a RJ-45 Ethernet jack connector. Both
of this connectors provide the asynchronous serial port SP-4 from the CPU to be used as a terminal’s
connection and the C50 enable line that serves to sense if the C50S/C50J connectors are being used or
not, this prevents the SP-4 port be used at the same time on the front connector and at A1 connector on
Serial motherboard.
SP-4 circuitry
The related circuitry consists of one RS-232 transceiver and two RS-485 transceivers that implement the
interface for transmitting and receiving data to and from the SP-4 coming from the CPU.
The RS-485 transceiver receives the SP-4 transmission differential signals coming from the P3 connector
and converts them to a single end signal; this signal enters to the RS-232 transceiver whose output is
connected to the Tx pin at the C50S/C50J connectors.
The Rx pin at C50S/C50J connectors receives the SP-4 reception signal and route it to the RS-232
transceiver whose output is connected to the RS-485 transceiver, then the differential signals are then
routed to the P3 connector.
The C50 enable circuit
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