PleasureCraft Engine Group 5.0L, Diagnostic Manual, Page: 23 / 337

5.0/5.7/6.0L/8.1L ECM and Sensors 2 - 3
MEFI 4 - PCM
4-18-91
MS 11696
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TIME
DIGITAL BINARY SIGNAL
Figure 2-3 - Digital Voltage Signal
Switch Types
Switched inputs (also known as discretes) to the ECM can
cause one bit to change, resulting in information being
communicated to the ECM. Switched inputs can come
in two types: “pull-up” and “pull-down” types. Both types
will be discussed.
With “pull-up” type switch, the ECM will sense a voltage
when the switch is CLOSED. With “pull-down” type switch,
the ECM will sense a voltage when the switch is OPEN.
Pulse Counters
For the ECM to determine frequency information from a
switched input, the ECM must measure the time between
the voltage pulses. As a number of pulses are recorded in
a set amount of time, the ECM can calculate the frequency.
The meaning of the frequency number can have any
number of meanings to the ECM.
An example of a pulse counter type of input is the
Crankshaft Position (CKP) sensor input. The ECM can
count a train of pulses, a given number of pulses per
engine revolution. In this way, the ECM can determine
the RPM of the engine.
Digital Signals
Digital signals are also variable, but not continuously. They
can only be represented by distinct voltages within a range.
For example, 1V, 2V or 3V would be allowed, but 1.27V or
2.56V would not. Digital signals are especially useful when
the information can only refer to two conditions: “YES” and
“NO,” “ON” and “OFF” or “HIGH” and “LOW.” This would
be called a digital binary signal. A digital binary signal is
limited to two voltage levels. One level is a positive voltage,
the other is no voltage (zero volts). As you can see in Figure
2-3, a digital binary signal is a square wave.
The ECM 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.
Binary code is used inside the ECM and between a
computer and any electronic device that understands the
code. By stringing together thousands of bits, computers can
communicate and store an infi nite varieties of information.
To a computer that understands binary, 11001011 might
mean that it should turn an output device “ON” at slow
speed. Although the ECM uses 8-bit digital codes internally
and when talking to another computer, each bit can have
a meaning.