Mercury MerCruiser MCM 454 Mag MPI Service Manual page 476

SERVICE MANUAL NUMBER 23
The computer uses digital signals in a code that contains only ones and zeros. The high volt-
age of the digital signal represents a one (1), and no voltage represents a zero (0). Each
zero and each one is called a bit of information, or just a "bit." Eight bits together are called
a "word." A word, therefore, contains some combination of eight binary code bits: eight ones,
eight zeros, five ones and three zeros, and so on.
Binary code is used inside a computer and between a computer and any electronic device
that understands the code. By stringing together thousands of bits, computers can
communicate and store an infinite variety of information. To a computer that understands
binary, 11001011 might mean that it should reset engine rpm at a lower level. Although the
computer uses 8-bit digital codes internally and when talking to another computer, each bit
can have a meaning.
SWITCH TYPES
Switched inputs (also known as discretes) to the computer can cause one bit to change,
resulting in information being communicated to the computer. Switched inputs can come in
two types: they are "pull-up" and "pull-down" types. Both types will be discussed.
With a pull-up type switch, the ECM will sense a voltage when the switch is CLOSED. With
the pull-down switch, the ECM recognizes the voltage when the switch is OPEN.
Discretes can also be used to inform a computer of FREQUENCY information.
PULSE COUNTERS
For the computer to determine frequency information from a switched input, the computer
must measure the time between voltage pulses. As a number of pulses are recorded in a
set amount of time, the computer can calculate the frequency. The meaning of the frequency
number can have any number of meanings to the computer.
An example of a pulse counter type of input is the distributor reference pulse input. The
computer can count a train of pulses, a given number of pulses per engine revolution, and
determine the rpm of the engine.
Index
90-861326--1 MARCH 1999
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MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
Page 5B-17