We have discussed, in the History of the Computer series, the operation and development of the computer and changes in the technology. The other components of a computer system, the peripherals, or external devices have also changed over the years. Drums have more or less disappeared, as have paper tape and punch cards. Monitors have appeared, replacing teletype machines as a way of operator communication. PAPER TAPE Mechanical paper tape readers used initially were too slow for the computer, and the sprocket hole was used to gate the data from the tape, read by photo-electric cells aligned across the tape path, to an input register, from where it was decoded and transferred to system memory. This required 9 sensors, one for each bit, and one for the sprocket. The reader pulled the tape past the sensors by holding it against a constant speed motor driven capstan with a rubber pinch roll, energised by a solenoid. The paper tape punch was still mechanical but could punch much faster than a typewriter driven punch. Maybe today we could use lasers to punch the holes, but we're past that now! PUNCH CARDS A medium used since the 1800s was adapted for use with computers, the punch card. These were accurately specified in terms of dimension and thickness and where the holes were punched. They had to be capable of being read at high speed. A common tool of the computer engineer was a 'card gauge'. A punched card could be placed on the gauge, and checked for accuracy. Thickness was important as the reader was set to take one card at a time, not 2. A box of cards, stored in a non air-conditioned environment was likely to be out of tolerance, and would be impossible to use. A program or data to be loaded into a computer would first be punched into the card by a key punch machine, an operator would type the data, one character at a time into the keyboard, and the machine would punch the appropriate pattern on the card. The deck of cards punched out could be fed through an 'interpreter', which would print the encoded information along the top of the card. Later, the card punch would incorporate an interpreter. The cards to be read into the computer would be stacked in a card reader, which read the data on the card in a similar way to the paper tape reader, with sensors. Card readers, along with everything else improved in speed reaching 2000 cards per minute. 'Everything else' included crashes, where a card got stuck in the read path. Some machines would stop dead, damaging a couple of cards, others were more spectacular, and sprayed them all over the computer room. A card punch was a standard computer peripheral device, and would punch the cards a row at a time, or a column at a time. This electro-mechanical beast typically operated at 100 cards per minute. In Part 2 we look at printers. |
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