The computer requires a means of storing both permanent and temporary memory. The memories contain many different locations. These locations can be compared to file folders in a filing cabinet, with each location containing one piece of information. An address is assigned to each memory location. This address may be compared to the lettering or numbering arrangement on file folders. Each address is written in a binary code, and these codes are numbered sequentially beginning with 0.
While the engine is running, the engine computer receives a large quantity of information from a number of sensors. The computer may not be able to process all this information immediately. In some instances, the computer may receive sensor inputs that the computer requires to make a number of decisions. In these cases, the microprocessor writes information into memory by specifying a memory address and sending information to this address. When stored information is required, the microprocessor specifies the stored information address and requests the information. When stored information is requested from a specific address, the memory sends a copy of this information to the microprocessor. However, the original stored information is still retained in the memory address.
The memories store information regarding the ideal air-fuel ratios for various operating conditions. The sensors inform the computer about the engine and vehicle operating conditions. The microprocessor reads the ideal air-fuel ratio information from memory and compares this information with the sensor inputs. After this comparison, the microprocessor makes the necessary decision and operates the injectors to provide the exact air-fuel ratio the engine requires.
FIGURE. EPROM memory is erased when the ultra-violet ray contact the microcircuitry.
Several types of memory chips may be used in the computer:
- Read only memory (ROM) contains a fixed pattern of l’s and O’s that represent permanent stored information. This information is used to instruct the microprocessor on what to do in response to input data. The microprocessor reads the information contained in ROM but it cannot write to it or change it. ROM is permanent memory that is programmed in. This memory is not lost when power to the computer is lost. ROM contains formulas, calibrations, and so on.
- Random access memory (RAM) is constructed from flip-flop circuits formed into the chip. The RAM will store temporary information that can be read from or written to by the pR RAM stores information that is waiting to be acted upon and it stores output signals that are waiting to be sent to an output device. RAM can be designed as volatile or nonvolatile. In volatile RAM, the data will be retained as long as current flows through the memory. RAM that is connected to the battery through the ignition switch will lose its data when the switch is turned off (see number 7, nonvolatile RAM).
- Keep alive memory (KAM) is a version of RAM. KAM is connected directly to the battery through circuit protection devices. For example, the microprocessor can read and write information to and from the KAM and erase KAM information. However, the KAM retains information when the ignition switch is turned off. KAM will be lost when the battery is disconnected, the battery drains too low, or the circuit opens.
- Programmable read only memory (PROM) contains specific data that pertains to the exact vehicle in which the computer is installed. This information may be used to inform the microprocessor of the accessories that are equimicroprocessored on the vehicle. The information stored in the PROM is the basis for all computer logic. The information in PROM is used to define or adjust the operating perimeters held in ROM.
- Erasable PROM (EPROM) is similar to PROM except that its contents can be erased to allow new data to be installed. A piece of Mylar tape covers a window. If the tape is removed, the microcircuit is exposed to ultraviolet light that erases its memory.
- Electrically erasable PROM (EEPROM) allows changing the information electrically one bit at a time. Some manufacturers use this type of memory to store information concerning mileage, vehicle identification number, and options. The flash EEPROM may be reprogrammed through the data link connector (DLC) using the manufacturer’s specified diagnostic equipment.
- Nonvolatile RAM (NVRAM) is a combination of RAM and EEPROM in the same chip. During normal operation, data is written to and read from the RAM portion of the chip. If the power is removed from the chip, or at programmed timed intervals, the data is transferred from RAM to the EEPROM portion of the chip. When the power is restored to the chip, the EEPROM will write the data back to the RAM.
Adaptive Strategy and Memory
If a computer has adaptive strategy capabilities, the computer can actually learn from past experience. For example, the normal voltage input range from an ambient temperature sensor may be 0.6 volt to 4.5 volts. If the sensor sends a 0.4-volt signal to the computer, the microprocessor interprets this signal as an indication of component wear and stores this altered calibration in the RAM. The microprocessor now refers to this new calibration during calculations and normal system performance is maintained. If a sensor output is erratic or considerably out of range, the computer may ignore this input. When a computer has adaptive strategy, a short learning period is necessary under the following conditions:
- After the battery has been disconnected.
- When a computer system component has been replaced or disconnected.
- On a new vehicle.
Adaptive memory is the ability of the computer system to store changing values in order to correct operating characteristics. For example, a transmission control module may monitor the transmission’s input and output shaft speeds to determine gear ratio. If the input speed sensor indicates a speed of 1,000 rpms and the output speed sensor indicates a speed of 333 rpms then the controller determines that the ratio is 3.00 to 1 (1st gear). When the controller determines that it will make the shift to second gear (2.00 to 1 ratio) it will monitor the sensors to see how long it takes to achieve the ratio change from 3.00 to 1 to 2.00 to 1. The length of time required represents the amount of fluid needed to stroke the clutch piston and lock up the clutch element. This value is learned so the timing of the shifts can be altered as the clutch elements wear, yet the quality of the shifts will not deteriorate over the life of the transmission.