Use of 74138 in ALS-SDA-85M kit

Microcontroller Microprocessor 8085

In the ALS-SDA-85M kit, we have 74138 EPROM of minimum size 16K x 8 and size of RAM IS 2K x 8, there are empty sockets for the EPROM to get expanded. The selection of these four chips is done by the integrated circuit 74138. Every time the selection of 74138 is done when IO/M* turns to 0, the election of 27138 is done when A15A14 turns to 0. 27138 have the lowest address range like

Lowest address: 00 00000000000000 is 0000H.

Highest address: 00 11111111111111 is 3FFFH.

Whereas in 74138 the address ranges are as follows -

Lowest address: 11 xxx 00000000000

Highest Address: 11 xxx 11111111111.

The outputs O1* and O2* gets connected to the output terminal of 74138.

The four chips get selected by using the four pins of the Integrated Circuit as drawn below -

The lowest and highest addresses for the 27128 is shown below –

Lowest address: 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 = 0 0 0 0 H

Highest address: 0 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 = 3 F F F F H

The lowest and the highest range of address of 6116 is described below –

Lowest address: 1 1 x x x 0 0 0 0 0 0 0 0 0 0 0

Highest address: 1 1 x x x 1 1 1 1 1 1 1 1 1 1 1

Updated on 30-Jul-2019 22:30:25

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