The air charge temperature (ACT) sensor input will be used as an example of how the computer processes information. If the air temperature is low, the air is denser and contains more oxygen per cubic foot. Warmer air is less dense and therefore contains less oxygen per cubic foot. The cold, dense air requires more fuel compared to the warmer air that is less dense. The microprocessor must supply the correct amount of fuel in relation to air temperature and density. An ACT sensor is positioned in the intake manifold where it senses air temperature. This sensor contains a resistive element that has an increased resistance when the sensor is cold. Conversely, the ACT sensor resistance decreases as the sensor temperature increases. When the ACT sensor is cold, it sends a high-analog voltage signal to the computer, and the A/D converter changes this signal to a digital signal.
When the microprocessor receives this ACT signal, it addresses the tables in the ROM. The look-up tables list air density for every air temperature. When the ACT sensor voltage signal is very high, the look-up table indicates very dense air. This dense air information is relayed to the microprocessor, and the microprocessor operates the output drivers and injectors to supply the exact amount of fuel the engine requires.
FIGURE. The microprocessor addresses the lookup tables in the ROM, retrieves air density information, and issues commands to the output devices.
FIGURE. The NOT gate symbol and truth table. The NOT gate inverts the input signal.
FIGURE. The AND gate symbol and truth table. The AND gate operates similar to switches in series.
FIGURE. The AND gate circuit.
FIGURE. OR gate symbol and truth table. The OR gate is similar to parallel switches.
FIGURE. Symbols and truth tables for NAND and NOR gates. The small circle represents an inverted output on any logic gate symbol.
FIGURE. XOR gate symbol and truth table. A XOR gate is a combination of NAND and NOR gates.
Logic gates are the thousands of field effect transistors (FETs) incorporated into the computer circuitry. These circuits are called logic gates because they act as gates to output voltage signals dependingon different combinationsof input signals. The FETs use the incomingvoltage patterns to determine the pattern of pulses leaving the gate. The following are some of the most common logic gates and their operations. The symbols represent functions and not electronic construction:
- NOT gate: A NOT gate simply reverses binary l’s to 0’s and vice versa. A high input results in a low output and a low input results in a high output.
- AND gate: The AND gate will have at least two inputs and one output. The operation of the AND gate is similar to two switches in series to a load. The only way the light will turn on is if switches A and В are closed. The output of the gate will be high only if both inputs are high. Before current can be present at the output of the gate, current must be present at the base of both transistors.
- OR gate: The OR gate operates similarly to two switches that are wired in parallel to a light. If switch A or В is closed, the light will turn on. A high signal to either input will result in a high output.
- NAND and NOR gates: A NOT gate placed behind an OR or AND gate inverts the output signal.
- Exclusive-OR (XOR) gate: A combination of gates that will produce a high-output signal only if the inputs are different.
FIGURE. Simplified temperature sensing circuit that will turn on the air conditioning compressor when inside temperatures reach a predetermined value.
FIGURE. Selection at inputs D, С, B, A will determine which data input will be processed.
FIGURE. Block diagram representation of the MUX and DEMUX circuit.
FIGURE. (A) RS flip-flop symbol. (B) Truth table. (C) Logic diagram. Variations of the circuit may include NOT gates at the inputs, if used, the truth table outputs would be reversed.
FIGURE. Clocked RS flip-flop symbol.
These different gates are combined to perform the processing function. The following are some of the most common combinations:
- Decoder circuit: A combination of AND gates used to provide a certain output based on a given combination of inputs. When the correct bit pattern is received by the decoder, it will produce the high-voltage signal to activate the relay coil.
- Multiplexer (MUX): The basic computer is not capable of looking at all of the inputs at the same time. A multiplexer is used to examine one of many inputs depending on a programmed priority rating. This process is called sequential sampling. This means the computer will deal with all of the sensors and actuators one at a time.
- Demultiplexer (DEMUX): Operates similar to the MUX except that it controls the order of the outputs.
- RS and clocked RS flip-flop circuits: Logic circuits that remember previous inputs and do not change their outputs until they receive new input signals. The illustration shows a basic RS flip-flop circuit. The clocked flip-flop circuit has an inverted clock signal as an input so that circuit operations occur in the proper order. Flip-flop circuits are called sequential logic circuits because the output is determined by the sequence of inputs. A given input affects the output produced by the next input.
- Driver circuits: A driver is a term used to describe a transistor device that controls the current in the output circuit. Drivers are controlled by the microprocessor to operate high-current circuits. The high currents handled by a driver are not really that high; they are just more than what is typically handled by a transistor. Several types of driver circuits are used on automobiles, such as Quad, Discrete, Peak and Hold, and Saturated Switch driver circuits.
- Registers: A register is a combination of flip-flops that transfer bits from one to another every time a clock pulse occurs. It is used in the computer to temporarily store information.
- Accumulators: Registers designed to store the results of logic operations that can become inputs to other modules.
FIGURE. It takes four clock pulses to load 4 bits into the register.