Memory Devices – IQ – 2

10. What is programmable logic array ? How it differs from ROM ? 
In some cases the number of don’t care conditions is excessive, it is more economical to use a second type of LSI component called a PLA. A PLA is similar to a ROM in concept ; however it does not provide full decoding of the variables and does not generates all the minterms as in the ROM.

11. What is mask – programmable ? 
With a mask programmable PLA, the user must submita PLA program table to the manufacturer. 

12. What is field programmable logic array ? 
The second type of PLA is called a  field programmable logic array.  The user by means of certain recommended procedures can programthe EPLA. 

13. List the major differences between PLA and PAL 
PLA : 
Both AND and OR arrays are programmable and Complex 
Costlier than PAL
AND arrays are programmable OR arrays are fixed 
Cheaper and Simpler 

14. Define PLD ?
Programmable Logic Devices consist of a large array of AND gates and OR gates that can be programmed to achieve specific logic functions. 

15. Give the classification of PLDs.
PLDs  are  classified  as  :
(1) PROM(Programmable  Read  Only Memory),
(2) Programmable Logic Array(PLA),
(3) Programmable Array Logic (PAL), and
(4) Generic Array Logic(GAL) 

16. Define PROM ?
PROM is Programmable Read Only Memory. It consistsof a set of fixed AND gates connected to a decoder and a programmable OR array.

17. Define PLA ?
PLA is Programmable Logic Array(PLA). The PLA is a PLD that consists of a programmable AND array and a programmable OR array.

18. Define PAL ?
PAL is Programmable Array Logic. PAL consists of aprogrammable AND array and a fixed OR array with output logic. 

19. Why was PAL developed ? 
It is a PLD that was developed to overcome certaindisadvantages of PLA, such as longer delays due to additional fusible links that  result from using two programmable arrays and more circuit complexity. 

20. Why the input variables to a PAL are buffered ?
The input variables to a PAL are buffered to prevent loading by the large number of AND gate inputs to which available or its complement can be connected. 

21. What does PAL 10L8 specify ? 
PAL – Programmable Logic Array 
10 – Ten inputs 
L – Active LOW Ouput 
8 – Eight Outputs 

22. Give the comparison between PROM and PLA.
PROM                                                            PLA 
1. And array is fixed and OR                     Both AND and OR arrays are 
array is programmable.                             Programmable. 
2. Cheaper and simple to use.                  Costliest  and  complex  than 

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