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Automotive Computer Basics

Gate Circuits

A computer Gate Circuit is simply a circuit that will allow or block the flow of electrons.
A Gate circuit has two or more inputs that will control an output in the circuit.

AND Circuit

In this circuit, both switch A AND switch B must be closed in order to turn
the lamp on. Notice in the Truth Table that 0=off and 1=on.

OR Circuit

In this circuit, switch A OR switch B can turn the light on.

NAND and NOR Circuits

Notice in the NAND (Not AND) and NOR (Not OR) circuits, the switches
are used to turn the output off. This means that the switches will allow
current to bypass to ground when the switches are closed.

Why Gate Circuits?

Gate circuits are used to enable computers to "make decisions". In reality, a computer is
really nothing more than a large number of transistor "switches" arranged so that when they
are activated in a certain pattern, they will switch current flow to the desired circuit.

For example, a soft drink machine has an AND circuit. When you place a coin in the slot
AND make a selection, the output is a can of whatever you have selected. This is achieved
simply by connecting two switches in series to complete the circuit.

Automotive Computer Inputs

In order for an Automotive Computer to successfully control the required systems,
certain INPUT information must be made available in the form of electrical signals.
Because a computer only understands 0=off and 1=on - any varying (analog) signals
must be changed to ON-OFF (digital) signals for the computer to understand them.

Input Conditioning

Notice that some signals must be amplified before they can be processed. For example, an
oxygen sensor produces a voltage less than 1.0 volts - and this must be increased
before the computer can use it to switch a transistor. Some signals, such as the throttle
position sensor, may be in the neighbourhood of 5.0 volts - and therefore don't
need to be amplified. From there - the analog (varying) signal must be changed to a
digital (on-off) signal so that the microprocessor can use it.

Inputs

Three types of inputs must receive a reference voltage from a special regulator in the
computer - this is usually 5.0 volts. The inputs that require a reference voltage are:

Variable resistors - that produce a voltage drop
Potentiometers - that vary the voltages at the end terminals
Switches - that open and close circuits

The other two types of inputs do not require a reference voltage, as they are
capable of producing their own voltages:

Voltage generators - that produce voltage through

various means

Magnetic pickups - that produce voltage through

electromagnetic induction

Computer Memory

There are 4 types of computer memory that are used to store information that
the circuits can reference to accurately control the required systems:

Fixed Memory

            ROM - Read Only Memory - is a fixed set of instructions (programmed switching
                                circuits) in the computer that can not be changed once in service. It is
                                permanent memory.

              PROM - Programmable Read Only Memory - is a removeable set of instructions
                                that may be changed in service by replacing an Integrated Circuit module
                                within the computer.

EPROM - Erasable Read Only Memory. This is erased by Ultraviolet light and
then can be reprogrammed.

              EEPROM - Electronically Eraseable Programmable Read Only Memory - allows the
                                ROM to be erased and reprogrammed while in service without removing
                                components from the computer. Most of today's vehicles have this
                                capability.

Temporary Memory

              RAM - Random Access Memory - this is temporary storage for information.
                                It remains intact as long as the battery inthe car is connected. If the
                                battery is disconnected, then the RAM is lost and has to be "relearned".
                                It is primarily used for storage of trouble codes.

              KAM - Keep Alive Memory - this is temporary storage that is lost whenever the
                                battery is disconnected - and serves to update information amd allow the
                                vehicle to make adaptations for wear and changing driving conditions
                                when the vehicle is running.

Computer System Outputs

A computer is composed of very small conductor circuits, and as a result, is not
capable of handling large current flows - including those in the ignition and fuel
systems. As a result, the computer must contain Output Drivers - which are transistors
that are controlled by the computer to switch current flow to higher demand devices
such as ignition coils and fuel injectors. These output drivers may, in turn, be used to
control relays or solenoids to complete high demand circuits.

Controlled Systems

Today's automotive computers are used to control:

Ignition systems - spark timing
Fuel systems - fuel pumps and injectors
Emissions - various emission control devices
Transmissions - automatic transmission shift points
Accessories - such as speed controls, climate controls,

entertainment systems, brake systems, active

suspensions, etc.

Because of this high demand for computer control, many
manufacturers are switching to a system of multiplexing - where
the various systems can interact and reduce the wiring demands
for separate inputs for each circuit.

Unit Three Assignment

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