Punched tape
Punched tape or paper tape is a largely obsolete form of data storage, consisting of a long strip of paper in which holes are punched to store data. It was widely used during much of the twentieth century for teleprinter communication, and later as a storage medium for minicomputers and CNC machine tools.
Origin
The earliest forms of punched tape come from weaving looms and embroidery, where cards with simple instructions about a machine's intended movements were first fed individually as instructions, then controlled by instruction cards, and later were fed as a string of connected cards. (See Jacquard loom).
This led to the concept of communicating data not as a stream of individual cards, but one "continuous card", or a tape. Many professional embroidery operations still refer to those individuals who create the designs and machine patterns as "punchers", even though punched cards and paper tape were eventually phased out, after many years of use, in the 1990s.
In 1846 Alexander Bain used punched tape to send telegrams.
Tape formats
Data was represented by the presence or absence of a hole in a particular location. Tapes originally had five rows of holes for data. Later tapes had 6, 7 and 8 rows. A row of narrower holes ("sprocket holes") that were always punched served to feed the tape, typically with a wheel with radial pins called a "sprocket wheel." Text was encoded in several ways. The earliest standard character encoding was Baudot, which dates back to the nineteenth century and had 5 holes. Later standards, such as Fieldata and Flexowriter, had 6 holes. In the early 1960s, the American Standards Association led a project to develop a universal code for data processing, which became known as ASCII. This 7-level code was adopted by some teleprinter users, including AT&T (Teletype). Others, such as Telex, stayed with Baudot.
The word "Wikipedia" as punched tape code (without a parity bit or with "spacing" parity) appears as follows:
File:Papertape-Wikipedia-example.png
Chadless Tape
Most tape-punching equipment used solid punches to create holes in the tape. This process inevitably creates "chad", or small circular pieces of paper. Managing the disposal of chads was an annoying and complex problem, as the tiny paper pieces had a distressing tendency to escape and interfere with the other electromechanical parts of the teletype equipment.
One variation on the tape punch was a device called a Chadless Printing Reperforator. This machine would punch a received teleprinter signal into tape and print the message on it at the same time, using a printing mechanism similar to that of an ordinary page printer. The tape punch, rather than punching out the usual round holes, would instead punch little U-shaped cuts in the paper, so that no chad would be produced; the "hole" was still filled with a little paper trap-door. By not fully punching out the hole, the printing on the paper remained intact and legible. This enabled operators to read the tape without having to decipher the holes, which would facilitate relaying the message on to another station in the network. Also, of course, there was no "chad box" to empty from time to time. A disadvantage to this mechanism was that chadless tape, once punched, did not roll up well, because the protruding flaps of paper would catch on the next layer of tape, so it could not be rolled up tightly. Another disadvantage, as seen over time, was that there was no reliable way to read chadless tape by optical means employed by later high-speed readers. However, the mechanical tape readers used in most standard-speed equipment had no problem with chadless tape, because it sensed the holes by means of blunt spring-loaded sensing pins, which easily pushed the paper flaps out of the way.
Applications
Communications
Punched tape was used as a way of storing messages for teletypewriters. Operators typed in the message to the paper tape, and then sent the message at the maximum line speed from the tape.
This permitted the operator to prepare the message "off-line" at the operator's best typing speed, and permitted the operator to correct any error prior to transmission. An experienced operator could prepare a message at 135WPM (Word Per Minute) or more for short periods.
The line typically operated at 75WPM, but it operated continuously. By preparing the tape "off-line" and then sending the message with a tape reader, the line could operate continuously rather than depending on continuous "on-line" typing by a single operator. Typically, a single 75WPM line supported three or more teletype operators working offline.
Tapes punched at the receiving end could be used to relay messages to another station. Large store and forward networks were developed using these techniques.
Minicomputers
When the first minicomputers were being released, most manufacturers turned to the existing mass-produced ASCII teletypewriters (primarily the ASR33) as a low-cost solution for keyboard input and printer output. As a side effect punched tape became a popular medium for low cost storage, and it was common to find a selection of tapes containing useful programs in most minicomputer installations. Faster, optical readers were also common.
Automated machinery
In the 1970s, computer-aided manufacturing equipment often used paper tape. Paper tape was a very important storage medium for computer-controlled wire-wrap machines, for example. A paper tape reader was smaller and much less expensive than hollerith card or magnetic tape readers. Premium black waxed and lubricated long-fiber papers, and Mylar "paper" tape were invented so that production tapes for these machines would last longer.
Cryptography
Paper tape was the basis of the Vernam cipher, invented in 1917. During the last third of the 20th century, the U.S. National Security Agency used punched paper tape to distribute cryptographic keys. The 8-level paper tapes were distributed under strict accounting controls and were read by a fill device, such as the hand held KOI-18, that was temporarily connected to each security device that needed new keys. NSA has been trying to replace this method with a more secure electronic key management system (EKMS), but paper tape is apparently still being employed.
Limitations
The three biggest problems with paper tape were
- Reliability. It was common practice to follow each mechanical copying of a tape with a manual hole by hole comparison.
- Rewinding the tape was difficult and prone to problems. Great care was needed to avoid tearing the tape. Some systems used fanfold paper tape rather than rolled paper tape. In these systems, no rewinding was necessary nor were any fancy supply reel, takeup reel, or tension arm mechanisms required; the tape merely fed from the supply tank through the reader to the takeup tank, refolding itself back into the exact same form as when it was fed into the reader.
- Low informaton density. Datasets much larger than a few dozen kilobytes are impractical to handle in paper tape format.
Advantages
Paper tape does have some useful properties:
- Longevity. Although many magnetic tapes have deteriorated over time to the point that the data on them has been irretrievably lost, punched tape can be read many decades later, probably outlasting many centuries.
- Human accessibility. The hole patterns can be decoded visually if necessary, and torn tape can be repaired (using special all-hole pattern tape splices).
Punched tape in art
A computing or telecommunications professional depicted in the Monument to the Conquerors of Space in Moscow (1964) holds what appears to be a punched tape with three rows of rectangular holes.
See also
- Bit bucket
- Book music
- Chad (the little pieces of paper punched out of the tape).
- Key punch
- Music roll
- Piano roll
- Punched card
- Zygalski (Perforated) sheets - a system used to decrypt messages enciphered on German Enigma machines.
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