Page 92 - Handout Digital Electronics

\Address Chip

0000-00FF 0

0100-01FF 1

0200-02FF 2
0300-03FF 3

So, each chip requires eight (8) address lines, and these are supplied by the bits 07-bitit lines. The
higher- order 2 bits, 8 and 9 are used to select one of the four chips by using the 2 to 4 decoders. The
outputs of the decoder enable only one of the four chips at a time.

Block diagram showing address decoding

Figure 24: Address decoding

The block diagram operates as follows:

To read or store data to a particular memory location, the 2 to 4 decoder is used to select the chip. For
example, if A8 = 0 and A9 = 0 then chip 0 will be selected by enable line 0 at the output of the decoder
to a 1. The address lines A0 –A7 are then used to identify the unique memory location where data can be
written to or read from. Only one chip operates at a time in the above arrangement.

11.2 Encoders
An encoder is a logic circuit that converts information or data from one form to another or from one
code to another, for example changing an octal code number to a binary number. The encoders are
exactly the opposite of decoders. In general, an encoder has m inputs and n output lines. Of the m input
line only one is activated at a time. Encoders can also be used for security (data inscription) and saving
memory space (data compression). An example of a basic encoder logic circuit is shown below:

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