Memory Devices – IQ – 1

Memory Devices » IQ – 1

1. List basic types of programmable logic devices ?
(1) Read only memory
(2) Programmable logic Array
(3) Programmable Array Logic

2. Explain ROM ?
A read only memory(ROM) is a device that includes both the decoder and the OR gates within a single IC package. It consists of n input lines and m output lines. Each bit combination of the input variables is called an address. Each bit combination that comes out of the output lines is called a word. The number ofdistinct addresses possible with n input variables is 2n.

3. Define address and word for ROM ?
In a ROM, each bit combination of the input variable is called on address. Each bit combination that comes out of the output lines is called a word.

4. State the types of ROM ?
(1) Masked ROM.
(2) Programmable Read only Memory.
(3) Erasable Programmable Read only memory.
(4) Electrically Erasable Programmable Read only Memory.

5. Explain PROM ?
PROM (Programmable Read Only Memory)
It allows user to store data or program. PROMs use the fuses with material like nichrome and polycrystalline. The user can blow these fuses by passing around 20 to 50 mA of current for the period 5 to 20µs. The blowing of fuses is called programming of ROM. The PROMs are one time programmable. Once programmed, the information is stored permanent.

6. Explain EPROM ?
EPROM(Erasable Programmable Read Only Memory)
EPROM use MOS circuitry. They store 1’s and 0’s as a packet of charge in a buried layer of the IC chip. We can erase the stored data in the EPROMs by exposing the chip to ultraviolet light via its quartz  window for 15 to 20 minutes.  It is not possible to erase selective information. The chip can be reprogrammed.

7. Explain EEPROM ?
EEPROM(Electrically Erasable Programmable Read OnlyMemory)
EEPROM also use MOS circuitry. Data is stored as charge or no charge on an insulated layer or an insulated floating gate in the device. EEPROM allows selective erasing at the register level rather than erasing all the  information since the information can be changed by using electrical signals.

8. 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.

9. What is RAM ?
Random Access Memory. Read and write operations can be carried out.

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
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
And array is fixed and OR array is programmable. Both AND and OR arrays are Programmable.
Cheaper and simple to use. Costliest and complex than
PROMS.
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