Computers And Logic Circuits

Dealing with computers can seem overwhelming for those who are accustomed to working with mechanical systems. Since we cannot actually see what is going on inside the computer or the system it controls, computers may not be as easy to understand as mechanical components such as transmissions and engines. However, computers are not as complicated as they might sound. This chapter will help demystify computers.

The computers found on a vehicle are really no different than any other computer encountered in everyday life. Vehicle computers rely on data from some type of input device and then follow the instructions in their programs to determine the required output. The input device may be a keyboard or a coolant temperature sensor, and the output may be video display or a fuel injector. The program the computer follows may be for word processing or for controlling fuel metering and engine timing.

Computers can process a great deal of data very quickly and accurately, making them very useful for several jobs including controlling many of the systems on an automobile. This chapter explains how a computer functions, starting with the inputs and outputs, the computer's central processing unit (CPU) and memory, and logic gates and their symbols.

Understanding how computers work is essential because most vehicles have some type of computer. Knowing how computers operate and fit together with various sensors and actuators will increase your ability to diagnose and repair problems.

This chapter is divided into the following sections:

Analog and Digital Inputs Analog and Digital Outputs Signals, including.

Analog and digital wave forms AID converters D/A converters Microprocessor

Random Access Memory (RAM) Read-Only Memory (ROM) Programmable Read-Only Memory

(PROM) Logic Circuits


As demonstrated in the previous chapter, the ECU, as well as any other automobile computer, depends on sensors to monitor various system functions and report their status back to the computer. Once the computer receives the data from the sensors, it analyzes it against preprogrammed standards and acts accordingly.

One problem with many of these inputs is that they do not speak the same language as the computer. The computer only understands digital signals or on/off signals. A resistive type sensor provides the computer

with a variable voltage, known as an analog signal. Some sensors, like the switch type sensors, do provide a digital signal for the computer. In this case, the computer can interpret the signal because it is either on or it is off-nothing in-between.

Because computers must have digital inputs to use the data received, all analog signals must be converted to digital. How computers interpret the analog signals with an A/D converter will be covered later.


Computer output to most actuators is digital. The signal tells the actuator to either turn on for a specified length of time or shut off. Stepper motors, relays and solenoids have only two modes of operation: on or off.

Again, when actuators require a variable voltage, such as the speed control for a blower motor for air conditioning, the computer needs another interpreter. In this case, the interpreter is a D/A converter, which will be covered later.


As explained previously, the two types of signals are analog and digital. The voltage of these signals may change slowly or very quickly depending on the sensor and what it monitors. When signals are expressed as wave forms on an oscilloscope, the analog signal shows up as a flowing line with curved peaks and valleys, indicating variable rises and drops in voltage. The digital signal has vertical rises and drops, and a horizontal line with sharp corners. The top horizontal lines indicate when the voltage is high or on and the bottom horizontal lines indicate when the voltage is low or off.

When using a voltmeter to measure digital or analog signals that change very quickly, such as speed sensor or RPM signals, it is important to remember that the meter reading is not a true representation of the signal. A voltmeter displays the average reading of the signal. For example, with a digital signal the voltmeter will display the average between zero volts (off) and the voltage when the circuit was on. The computer looks for "on" signals, not voltage. The voltmeter, however, is looking for voltage, not whether a signal comes through. A voltmeter may show that the voltage is within specifications even if a pulse is missing. That missing signal could represent the cause of an engine problem. You might not know it by the voltmeter, causing you to assume incorrectly the problem is elsewhere and waste time searching.

So if you suspect the problem is in a certain circuit, but the voltmeter does not show it, consider using an oscilloscope for a more accurate reading. At the very least, you should be aware of this voltmeter limitation with digital signals.

When dealing with computer signals it is also important to remember that there is a difference between the signal source and the source of the voltage on the signal wire. This is especially important when a sensor input goes to more than one computer, such as a speed sensor signal, or if the signal is from one computer to another. One computer may supply the voltage to the sensor which toggles the voltage to ground, and the other computer may just monitor the signal. If a wire is disconnected from the computer that supplies voltage to the sensor, the signal is lost to both computers. Do not mistake this for a defective computer.

Analog signals also have limitations in that their inputs are not usable by the computer until translated into digital signals. The A/D converter handles that translation.

This takes us briefly back to computer language. Digital on/off can be represented by the binary numbering system of 0 (off) and 1 (on). Any decimal number (1, 2, 3, etc.) can be represented using O's and 1's so the computer understands. The several thousand transistors inside the computer's microprocessor can switch on and off in combinations that equal any binary number in a microsecond.







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