Programmable Read Only Memory (PROM) can be used as a programmable logic device or memory. As a programmable logic device, PROM uses AND gates as inputs to generate all possible combinations, and programmable OR gates to select the desired output combinations. It has 2n AND gates and m OR gates, where n is the number of inputs and m is the number of outputs. An example shows how an 8x2 PROM can be used to implement two Boolean functions of three variables. PROM can also be used as a memory device, where it stores m-bit data words at 2n address locations.

1. Programmable Read Only Memory
(PROM)
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Syed Hasan Saeed, Integral University,
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3. Programmable Read Only Memory (PROM)
• The Programmable Read Only Memory (PROM) enables the user
to implement the digital circuits (hardware) to execute a random
combinational function of a given number of input variables.
• When PROM is used as a memory device then ‘n’ number of
address lines store ‘m’ bit of data word each. Total memory size is
2n * m bits.
• When PROM is used as Programmable Logic Device (PLD) then
PROM can be used for implementing the functions of ‘n’ variables
to execute in ‘m’ number of ways.
• The general device has 2n hard-wired AND gates at input and ‘m’
number of programmable OR gates at output. Each AND gate has
‘n’ inputs and each OR gate has 2n inputs.
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Syed Hasan Saeed, Integral University,
Lucknow
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4. • For a given number of input variables, AND gates creates all conceivable
minterms and programmable OR gates permits only desired minterms to
appear at their inputs.
• Fig. 1 shows two input lines, four hard-wired AND gates and two
programmable OR gates. A cross ( ) indicates the unprogrammed
fusible link and dot ( ) indicates hard-wired connections.
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Syed Hasan Saeed, Integral University,
Lucknow
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Fig. 1
hard-wired connection
intact fusible linkAND
Gates
OR gates
TWO
OUTPUTS
TWO
INPUTSA B

5. PROGRAMMABLE ROMs AS PROGRAMMABLE LOGIC DEVICE:
• Now consider PROM as a programmable logic device for implementing the
combinational logic functions.
• PROM has ‘n’ inputs and ‘m’ output lines.
• It is combinational circuit which has AND gates at input and equivalent to
decoder and OR gates equal to the number of outputs.
• Consider the following Boolean functions
F1 (A,B,C) = ∑ (0,1,3)
F2 (A,B,C) = ∑ (2,4,6)
• Fig. 2 shows the architecture of 8 x 2 PROM. At input side hard-wired AND
gates creates all possible 8 minterms (product terms) for three variables. All
minterms are accessible at the inputs of each OR gates through
programmable interconnections.
• An unprogrammed interconnection is shown by (cross ) is a ‘make’
connection.
• Generally PROM is used for complex Boolean functions.
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Syed Hasan Saeed, Integral University,
Lucknow
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6. 8 X 2 PROM:
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Syed Hasan Saeed, Integral University,
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A CB
F1
F2
F1 (A,B,C) = ∑ (0,1,3)
F2 (A,B,C) = ∑ (2,4,6)
Fig. 2
3 X 8
DECODER

7. EXAMPLE: Design a combinational circuit using PROM to convert
gray code into binary code.
SOLUTION:
STEP 1: Truth Table for Grey to Binary code
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Syed Hasan Saeed, Integral University,
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GRAY INPUT
BINARY OUTPUT
(Y)
G2 G1 G0 B2 B1 B0
0 0 0 0 0 0
0 0 1 0 0 1
0 1 1 0 1 0
0 1 0 0 1 1
1 1 0 1 0 0
1 1 1 1 0 1
1 0 1 1 1 0
1 0 0 1 1 1

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Step 2: Simplification by using K-map:
0 0 0 0
1 1 1 1
0 0 1 1
1 1 0 0
0 1 0 1
1 0 1 0
00 01 11 10
00 01 11 10
00 01 11 10
0
0
0
1
1
1
G2 G1 G0
G2 G1 G0
G2 G1 G0
-(1)-----GB 22 
-(2)-----GGGGB 12121 
(3)-----GGGGGGB 0120120 

9. STEP 3:
• No. of inputs = 3
• No. of address lines (locations) = 23 = 8
• Each location can store 4 bit words
• No. of outputs = 4 ( B3, B2, B1, B0)
ROM Programming Table: (with the help of Eq. 1, 2 and 3)
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Syed Hasan Saeed, Integral University,
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Location
No.
Inputs Outputs
G2 G1 G0 B3 B2 B1 B0
0 0 0 0 0 0 0 0
1 0 0 1 0 0 0 1
2 0 1 0 0 0 1 1
3 0 1 1 0 0 1 0
4 1 0 0 0 1 1 1
5 1 0 1 0 1 1 0
6 1 1 0 0 1 0 0
7 1 1 1 0 1 0 1

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0
1
2
3
4
5
6
7
3*8
DECODER
G2
G1
G0
B3 B2 B1 B0
FIG 3: LOGIC
DIAGRAM Binary Output
Gray
Input
0000
0001
0011
0010
0111
0110
0100
0101

11. THANK YOU
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Syed Hasan Saeed, Integral University,
Lucknow
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