271 Read Only Memories

The program that controls the system is stored in the ROM, or read only memory. Various types of ROM are used in automotive computers and it is important to understand the differences between them because a service procedure that is used on one make of vehicle may not necessarily work on a similar type of vehicle. Part of the reason for this is to be found in the different types of ROM that are used.

Figure 2.8 shows a block diagram of a small ROM circuit. The truth table shows the data that appears on the output lines (Fo to F3). It is read as follows: when the electrical inputs at A, B and C represent logic 0, the output values F0 to F3 are 1010, and so on.

ROM circuit 8 words, each 4 bits

ROM circuit 8 words, each 4 bits

F, F2 Output lines

Inputs ABC

Fo

Outputs F, F2

F3

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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