A card reader is a device used to reads the information punched into a card, converting the availability or non availability of a hole into an electrical signal representing by a binary number 0 or 1. Thus the holes in a card are converted into coded electrical pulses which may be accepted by CPU.
Mechanism of Card Reader :-
As the card moves from hopper to the stacker, it passes through the sensing station. At this sensing station, either wire brushes or photoelectric cells which detect the presence or absence of holes. After the data is read at the first sensing station then the card passes through the second sensing station where data is read again. The data read at the two stations are then compared to verify the accuracy of input operations. If the two reads match, the data is accepted by the card reader. If there is any mismatch between two reads , the card reader stops the working and error message is given to the operator automatically.
The speed of card reader vary from 300 to 2000 cards per minute. Thus for an 80-column card reader which operates at a speed of 2000 cards per minute, the transfer rate of data will be 80 x 2000 or 160,000 character per minute. This transfer rate of data is not even one percent of the data transfer rates of the magnetic devices such as magnetic tape and disc.
Advantages of Card Reader :-
1.Machine interpretation of data is not always necessary in case of cards.
2.Cards are an old and liable medium. They remain a visible means of entering amount of data into computer systems.
3.By simply throwing the incorrect card and replacing it with a correct card, any wrong instruction or data in a card-deck can be changed easily but in case of punched-paper tape, magnetic disc and magnetic tape it is not easy.
Limitations of Card Reader :-
1.Cards can't be erased and used to enter new data.
2.Folded or stapled cards are not acceptable by a card reader. So card must be stored properly.
3.Cards are not suitable for the storage of voluminous data because of low data density and bulky card files.