Digital Signals - Mercury MerCruiser D2.8L D-Tronic Service Manual

SERVICE MANUAL NUMBER 22

Digital Signals

GENERAL
The computer uses digital signals in a code that contains only ones and zeros. The high
voltage 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.
PROCESSING THE DIGITAL INPUTS
The digital inputs are centrally read, processed and distributed throughout the system. The
first input message indicates the raw electrical conditions; a special internal message
indicates the logical, validity-checked conditions of the inputs.
Digital input signals that are processed:
1. Glow time feedback
2. Low-idle switch
3. Terminal 15 (battery voltage, key ON)
4. Terminal 15, not validity-checked
5. Logical conditions, validity-checked
6. Electrical conditions, not validity-checked
Inputs that are not used are masked out. Each input signal is checked for validity
(correctness). Every input is converted into its assigned logical level dependant on four data
set parameters.
The four parameters for every input:
a. Maximum
b. Off-Limit
c. On-Limit
d. Minimum
For Terminal 15 input, the unchecked (not validity checked) status is also made available.
90-860074--1 FEBRUARY 2002
DESCRIPTION
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