AE–1800 Instruction Manual l
Theory of Operation
8. Summarized T heory of Operation
(
continued
–
4 /10
)
8.7. Operat ing Principle of AE–1800
(
continued –2 / 2
)
CPU Circuit
The Main PCB (M613–MAIN–X) carries a CPU circuit and a power supply circuit. The
CPU is a 16–bit single–chip microcomputer (U10) clocked at 24 MHz by
crystal–controlled oscillator X1, and on–chip flash ROM for program software and
on–chip RAM providing memory space for data processing of the demodulated signals.
An external static RAM (SRAM) chip (U5, 512 kB) is mounted for the storage of
messages and IDs, and is backed up by large capacity capacitor C6 (1.0 farad) for
approx. 10 days in the event the unit is placed in a continuous power–off condition. A
portion of the memory capacity is allocated as a video RAM (VRAM) to hold text for
display on the LCD screen.
U3 is factory–programmed to act as a display controller to perform various display
control functions and text scrolling. The input/output interface between the CPU and
externally connected devices consists of three external ports: U4 for the
RS– 232C
and
I / O DATA
ports, U12 for the
RS– 422
port.
Power Supply Circuit
The power supply circuit consists mainly of a switch–mode voltage regulator (U55), a
power transformer (T51), a DC–to–DC converter (U52) (or U57 3–terminal voltage
regulator) and a 3–terminal voltage regulator (U56), producing various regulated
outputs (3.3V for IC chips,
±
12V for RS–232C port, +12V for the receiver PCB, +5V for
RS–422 port, etc.) required for operation of the entire Main PCB circuitry.
The voltages (+8V and +5V) required for receiver operation are derived from the +12V
output through 3–terminal regulators (U7 and U8) on the receiver board.
Should the power line polarity be reversed accidentally, diode D51 will cause the fuse F1
(3A) to blow, thereby protecting the input circuit from possible damage. A varistor
(VA51) is placed across the DC input line to protect the power supply circuit from
possible damage in the event voltage transients on the ship’s power line voltage exceed
40V.
