Addressing 5 ROM Chips, Each of 4 KB Using 74LS138 Decoder

Addressing 5 ROM Chips, Each of 4 KB Using 74LS138 Decoder:

The following diagram is representing the addressing of 5 ROM chips using 74LS138 where each chip is of 4 KB.

Explanation: From the previously given figure 01 it can be found that the Chip Select (CS) of the 5 ROM chips has been attached with the 74LS138. 8 data outputs of each chip have been connected to 8 data bus. As each ROM is of 4 KB or 212 B, each of them has been connected A12, A13, A14 to 12 address bus, from A0  to A11. 3-bits address A, B, C have been connected to  respectively to get select inputs. Now, when the 74LS138 will be enabled by setting G2A & G2B to low and G1 to high, by the value of A, B, C a ROM will be selected to read.  Now, how each location of each ROM will be selected can be determined by the following given worksheet or table;

 215A15 214A14 213A13 212A12 211A11 210A10 29A9 28A8 27A7 26A6 25A5 24A4 23A3 22A2 21A1 20A0 ROM0 Start 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 End 0 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 ROM1 Start 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 End 0 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 ROM2 Start 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 End 0 0 1 0 1 1 1 1 1 1 1 1 1 1 1 1 ROM3 Start 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 End 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 ROM4 Start 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 End 0 1 0 0 1 1 1 1 1 1 1 1 1 1 1 1

So, from the above table it can be found that, by setting the value of
A15 to 0 we are enabling 74LS138, by the values of A12, A13, A14  we are selecting that which ROM will be read, then for each location of that ROM we are having 12 digit binary value where the value for first location is 000000000000 and value for the last location is 111111111111.

Author: Tanmay Chakrabarty

Tanmay Chakrabarty is a former CSE student, currently working as a Senior Software Engineer with 5+ years of experience in the field of Web Application development in PHP+MySQL platform with strong skills in Javascript, JQuery, JQuery UI and CSS. He tries to write notes every week but fails due to heavy loads of duty.