first of these is the word or single word. A word
contains exactly the same number of bits as the
computer’s registers (16, 30,32, or 64bits). In 16-bit
microprocessors with 8-bit memory words, a 16-bit
word is assembled from two bytes of memory (fig.
4-42). The word forms the basis for most operand/data
operations in 16-bit and larger word size computers.
l Double word —Large numbers are often a
problem in digital computers. There are a number of
mathematical operations in which the size of the result
would be greater than the length of either of the two
registers used to provide inputs to the arithmetic logic
unit (ALU) or the operands being input to the ALU are
larger than a single word. For these situations, double
length memory words or double words are often used
in computers. A double word is an addressable data
element that can be stored in memory (two sequential
memory words), or loaded into registers (two sequential
registers), and used as an operand for mathematical
operations dealing with extremely large numeric
values.
TOPIC 8—POWER SUPPLIES
All digital computers have an internal power
supply. The power supply in the computer does not
supply power. It receives ac voltage from a source and
converts it into useable dc voltage(s). Most computers
require multiple dc voltages and levels. The dc is then
distributed to where it is needed. The power supply in
a computer is a switching power supply. This means
the power supply can handle quite a range of power
supply irregularities with minimal difficulties. It is
designed to provide precision voltages, sense
irregularities (input and output), and protect the
computer from serious damage. Let’s see how the
Figure 4-42.—Assembling a 16-bit word from two bytes.
computer’s power supply accomplishes this; we begin
with the operations.
The basic operation of any computer’s power
supply is accomplished by four basic sections: a
transformer, a rectifier, a filter, and a regulator.
How each computer performs this operation depends
on the computer’s requirements. Figure 4-43 is a block
diagram of a basic power supply. Because of their
general makeup, digital computers use power supplies
and, in some cases, external devices that allow the
power supply to provide precision voltage and internal
protection. The four basic sections of a power supply
make up the foundation used to provide additional
circuitry. The computer will receive the precision
voltage and protection. For a detailed description of
power supply operations in general, consult NEETS,
Module 7, Introduction to Solid-State Devices and
Power Supplies. For a detailed description of your
computer’s power supply, consult its technical manual.
INPUT
The computer can handle a range of input voltages
and frequencies. The computers aboard ship receive
their power from a main switch board via a load
center(s), a power panel(s), and outlets.
WARNING
SHIPS USE AN UNGROUNDED
ELECTRICAL DISTRIBUTION SYS-
TEM; THEY ARE DEADLY. BE SAFE,
KNOW YOUR SOURCE OF POWER.
Computers ashore receive power from a centralized
source, and the power is distributed via power panels
and outlets. The different ranges depend on the type of
computer and/or where the computer is used. These
inputs include:
. 440 vac, 60 Hz, 3 phase A
. 115 vac, 60 Hz, 1 phase A
. 115 vac, 60 Hz, 3 phase A
Figure 4-43.—Block diagram of a basic power supply.
4-25
