Garmin 155XL Maintenance Manual - page 9

190-00067-25 Rev D 

  1-3

1.3.1

Control Display Unit

The Control/Display Unit (CDU) is an assembly consisting of eleven keycaps, keypad board, optical lens, liquid

crystal display and board with heat sealed drivers, photocell and screws.  All components are housed in a die cast

bezel with a dual concentric rotary switch and knobs.  There are two LED’s behind each keycap, providing

backlighting for nighttime use.

Figure 1-2. GPS 155XL Control/Display Unit Block Diagram

1.3.2

Main Board Assembly

The Main Board Assembly consists of the following:

Central Processing Unit (CPU)

Operating system Read-Only Memory (ROM)

System memory

Discrete Input/Output (I/O)

Serial communication drivers and receivers

Power supply

These items are discussed in detail in the following paragraphs.  Figure 1-3 shows the Main Board Assembly

block diagram.

1.3.2.1 CPU Board

The CPU Board is a microprocessor-based computer board.  This board contains an Intel 80L186EB

microprocessor running at 16 MHz, a plug-in read only memory chip (ROM) and random access memory chips

(RAM).  Data stored in RAM is maintained by a 3-V lithium battery when the unit is switched off and by the

regulated 5-V supply when the unit is powered on.  A custom large scale integrated circuit (LSI) is used to

decode signals from the GPS satellites.  A real time clock Integrated Circuit (IC) is used to keep track of the date

and time.