An electrically erasable PROM works in a similar way to the EPROM, the main difference is that the memory can be erased and reset by a special charge pump circuit that is controlled by the microcontroller according to the working program of the ECM.


The RAM, or random access memory, is the section of memory that is used for temporary storage of data while it is being worked on. It must be the type of memory that can be written to (i.e. data is placed in it) and read from (i.e. data is taken from it). This means that the memory contents are constantly changing whenever the computer is operating. It is also known as a read-and-write memory. The contents of RAM are sustained by electricity and when the source of electrical power is removed, the contents of the RAM are lost. This is why the RAM is called volatile memory.


Hard discs are made of material that can be magnetized in very small localized areas arranged in circular tracks. A magnetized area probably represents a '1' in computer language, and a non-magnetized area represents a '0'. When the disc is rotated through a read-and-write head, the magnetism is converted into an electrical signal and it is these electrical signals that produce the data that operates the computer. Hard discs can hold many millions of bits of data. Floppy discs operate on similar principles, but they have a smaller storage capacity.

Compact discs (CDROMs) also have a large storage capacity. They are similar to an old fashioned gramophone record in that they have grooves which are deeper in some places than they are in others. The depth of the groove is read by a laser beam that is connected to an electronic circuit and this circuit converts the laser readings into voltages that represent the 0s and 1s used by the computer.

2.8 Fault codes

When a microcontroller (computer) is controlling the operation of an automotive system, such as engine management, it is constantly taking readings from a range of sensors. These sensor readings are compared with readings held in the operating program and if the sensor reading agrees with the program value in the ROM, the microcontroller will make decisions about the required output to the actuators, such as injectors.

If the sensor reading is not within the required limits it will be read again and if it continues to be 'out of limits' a fault code will be stored in a section of RAM. It is also likely that the designer will have written the main program so that the microcontroller will cause the system to operate on different criteria until a repair can be made, or until the fault has cleared. The fault codes, or diagnostic trouble codes (DTCs), are of great importance to service technicians and the procedures for gaining access to them need to be understood. It should be clear that if they are held in ordinary RAM, they will be erased when the ECM power is removed. This is why various methods of preserving them are deployed.

The term keep alive memory (KAM) refers to the systems where the ECM has a permanent, fused, supply of electricity. Here the fault codes are preserved, but only while there is battery power. Figure 2.9 shows a circuit for a KAM system.

Fig. 2.9 A KAM system

EEPROMs are sometimes used for the storage of fault codes and other data relating to events relating to the vehicle system. This type of memory is sustained even when power is removed. The use of fault codes is discussed in Chapter 3.

2.9 Adaptive operating strategy of the ECM

During the normal lifetime of a vehicle it often happens that compression pressures and other operating factors change. To minimize the effect of these changes, many computer controlled systems are programmed to generate new settings that are used as references, by the computer, when it is controlling the system. These new (learned) settings are stored in a section of memory, normally RAM. This means that such 'temporary' operating settings can be lost if electrical power is removed from the ECM. In general, when a part is replaced or the electrical power is removed for some reason, the vehicle must be test driven for a specified period in order to permit the ECM to 'learn' the new settings. It is always necessary to refer to the repair instructions for the vehicle in question, because the procedures do vary from vehicle to vehicle.


When a defect occurs that affects the engine, but is not serious enough to prevent the vehicle from being driven, the ROM program will normally contain an alternative loop in the program that will allow the vehicle to be driven to a service point. This mode of operation is often referred to as the 'limp home mode'. It should be noted that any attempt to cure a defect must take account of the fact that the system may be in its limited operating mode.

2.10 Networking of computers

As the use of separate computer controlled systems has increased, the desirability of linking the systems together has become evident and it is now quite common to find systems, such as engine management, traction control, anti-lock braking etc., working together to produce improved vehicle control. When computer controlled systems are linked together they are said to be 'networked'. The networking of computers on vehicles is often referred to as 'multiplexing', but as an introduction to the topic it is helpful to consider some general principles of computer networking as this provides a good insight into the networking that is used on vehicles.


Figure 2.10 shows the basic principle of a number of computers which are linked together by a common wire along which are sent the messages that the computers use to share data.

A principal advantage of this system is that it reduces the number of wires that are needed but, as you will appreciate, there are likely to be problems if more than one message is 'on the data bus' at any one time. The problem is overcome by having strict rules about the way in which data is moved between the computers connected to the bus. These sets of rules are known as 'protocols'.


An alternative to the bus system of connecting computers together is the star system shown in Fig. 2.11. An advantage of this system is that a break in the

Fig. 2.10 A simple bus-based network of computers
Fig. 2.11 A star-connected computer network

connection between one computer and the hub will not cause a failure of the entire network. The central hub can also be an electronic switch that receives messages from any of the computers. It then determines which of the computers (ECMs) on the network is the intended destination and then sends the message to that computer (ECM) only.

Networked computer systems of both types are often found on the same vehicle. Controller area network (CAN) is a networking system devised by Bosch that is widely used for high speed automotive networks. (Note, the term 'high speed' refers to the speed at which data is moved around the network and not the speed of the vehicle.) When a vehicle is equipped with networks that operate at different speeds (baud rates, or data bits per second), the normal practice is to permit them to communicate with each other via an interface known as a 'gateway'.

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