Remembering the basics of electricity, voltage does not flow through a conductor; current flows and voltage is the pressure that “pushes” the current. However, voltage can be used as a signal; for example, difference in voltage levels, frequency of change, or switching from positive to negative values can be used as a signal.
The computer is capable of reading only voltage signals. A program is a set of instructions the computer must follow to achieve desired results. The program used by the computer is “burned” into integrated circuit (1С) chips using a series of numbers. These numbers represent various combinations of voltages that the computer can understand. The voltage signals to the computer can be either analog or digital. Many of the inputs from the sensors are analog variables. For example, ambient temperature sensors do not change abruptly. The temperature varies in infinite steps from low to high. The same is true for several other inputs such as engine speed, vehicle speed, fuel flow, and so on.
FIGURE. Analog voltage signals are constantly variable. Digital voltage patterns are either on or off. Digital signals are referred to as a square sine wave.
FIGURE. Simplified voltage sensing circuit that indicates if the switch is opened or closed.
Compared to an analog voltage representation, digital voltage patterns are squareshaped because the transition from one voltage level to another is very abrupt. A digital signal is produced by an on/off or high/low voltage. The simplest generator of a digital signal is a switch. If 5 volts are applied to the circuit, the voltage sensor will read 5 volts (a high-voltage value) when the switch is open. Closing the switch will result in the voltage sensor reading close to 0 volts. This measuring of voltage drops sends a digital signal to the computer. The voltage values are represented by a series of digits, which create a binary code. Binary code is represented by the numbers 1 and 0. Any number and word can be translated into a combination of binary l’s and 0’s.
A transistor that operates as a relay is the basis of the digital computer. As the input signal switches from off to on, the transistor output switches from cutoff to saturation. The on and off output signals represent the binary digits 1 and 0.
The computer converts the digital signal into binary code by translating voltages above a given value to 1 and voltages below a given value to 0. As shown, when the switch is open and 5 volts are sensed, the voltage value is translated into a 1 (high voltage). When the switch is closed, lower voltage is sensed and the voltage value is translated into a 0. Each 1 or 0 represents one bit of information.
FIGURE. Each binary 1 and 0 is one bit of information. Eight bits equal one byte.
FIGURE. Binary number code conversion to base 10 numbers.
In the binary system, whole numbers are grouped from right to left. Because the system uses only two digits, the first portion must equal a 1 or a 0. To write the value of 2, the second position must be used. In binary, the value of 2 would be represented by 10 (one two and zero ones). To continue, a 3 would be represented by 11 (one two and one one). Figure illustrates the conversion of binary numbers to digital base ten numbers. If a thermistor is sensing 150 degrees, the binary code would be 10010110. If the temperature increases to 151 degrees, the binary code changes to 10010111.
The computer contains a crystal oscillator or clock circuit that delivers a constant time pulse. Hie clock is a crystal that electrically vibrates when subjected to current at certain voltage levels. As a result, the chip produces a very regular series of voltage pulses. The clock maintains an orderly flow of information through the computer circuits by transmitting one bit of binary code for each pulse. In this manner, the computer is capable of distinguishing between the binary codes such as 101 and 1001.
FIGURE. Interaction of the main components of the computer. All of the components monitor clock pulses.
Signal Conditioning and Conversion
The input and/or output signals may require conditioning in order to be used. This conditioning may include amplification and/or signal conversion.
Some input sensors produce a very low-voltage signal of less than 1 volt. This signal also has an extremely low-current flow. Therefore, this type of signal must be amplified, or increased, before it is sent to the microprocessor. This amplification is accomplished by the amplification circuit in the input conditioning chip inside the computer.
FIGURE. Amplification and interface circuits in the computer. The amplification circuit boosts the voltage and conditions it. The interface converts analog inputs into digital signals. The digital-to-analog converter changes the output from digital to analog.
For the computer to receive information from the sensor and give commands to actuators, it requires an interface. Hie computer will have two interface circuits: input and output. An interface is used to protect the computer from excessive voltage levels and to translate input and output signals. The digital computer cannot accept analog signals from the sensors and requires an input interface to convert the analog signal to digital. The analog to digital (A/D) converter continually scans the analog input signals at regular intervals. For example, if the A/D converter scans the TPS signal and finds the signal at 5 volts, the A/D converter assigns a numeric value to this specific voltage. Then the A/D converter changes this numeric value to a binary code.
FIGURE. The A/D converter assigns a numeric value to input voltages and changes this numeric value to a binary code.
Also, some of the controlled actuators may require an analog signal. In this instance, an output digital to analog (D/A) converter is used.