标签: teleprinter

Note: This material is abstracted from the books I co-authored with my friend Alvin Brown: Bebop Bytes Back (An Unconventional Guide to Computers) and How Computers Do Math . Users of the early computers required some kind of reliable, cheap, and efficient media for storing and transporting large amounts of computer data. Two techniques that became very widely used in the early days of computing were paper tapes and punched cards... Perforated paper products As you can imagine, it would be somewhat inconvenient to have a computer that forgot everything it knew if its operator turned it off before stepping out for a bite of lunch. Similarly, imagine a programmer's frustration if, after spending countless hours using a Switch Panel to enter a program, the janitor carelessly disconnected the computer in order to vacuum the office. A few choice words would be the order of the day, let me tell you! There were also a variety of other considerations. For example, since the early computers had very little memory anyway, it was necessary to have some mechanism to store large amounts of data outside of the machine's main memory. A program could then access and process small portions of the data on an as-needed basis. Also, if the operator had a number of different programs, but there wasn't enough memory to contain them all at the same time, then it was necessary to have some technique for storing the inactive programs. There was also the question of long-term archival; that is, being able to store programs and data for use sometime in the future. Yet another concern was being able to transport programs and data between computers located at different sites. For example, a programmer who created an interesting routine in Boston may well have wanted to share the fruits of his or her labors with colleagues in San Francisco. Although some flavours of early memories, such as Magnetic Core Stores, were non-volatile (they remembered their data when power was removed from the system), it was still somewhat less than practical to slip something the size of a large washing machine into an envelope and drop it into the mail. For all of these reasons, it was obvious to everyone that it would be advantageous to have some kind of reliable, cheap, and efficient media for storing large amounts of computer data (and preferably something that weighed-in at substantially less than a ton). In order to satisfy these requirements, two techniques became very widely used: paper tapes and punched cards, both of which involved perforating paper-based products (try saying that ten times quickly). The origin of paper tapes By using paper tapes, like so many other aspects of computing, engineers took advantage of technology that already existed at the time. In 1837, the British physicist and inventor Sir Charles Wheatstone and his friend, the British electrical engineer Sir William Fothergill Cooke invented the first British electric telegraph. Sir Charles was a busy man. Amongst other things, he also invented the accordion in 1829 (following which, presumably, he didn't have too many friends left) and three-dimensional photographs in the form of his Stereoscope in 1838. Apropos of nothing at all, 1837 was also the year that another "Charles" – Charles Dickens – first published a story under his given name; prior to this he'd been using the pen-name "Boz". But we digress.... Wheatstone's first telegraph made use of five wires, each of which was used to drive a pointer at the receiver to indicate different letters. In the same year, the American inventor Samuel Finley Breese Morse developed the first American telegraph, which was based on simple patterns of "dots" and "dashes" called Morse Code being transmitted over a single wire. Morse's system was eventually adopted as the standard technique, because it was easier to construct and more reliable than Wheatstone's.   Subset of International Morse Code The duration of a "dash" is three times the duration of a "dot". Note that the illustration above shows only a subset of the code (although it's quite a large subset), but it's enough to give the general idea. Also note that this table shows International Morse Code , which is a slightly different flavour to American Morse Code . Morse Code has a number of interesting features and, knowing me, you'll be lucky to escape without my mentioning at least a few of them. One tasty little nugget of trivia I simply can't resist pertains to the code for the letter "V". In his early years, Morse was more attracted to the arts than he was to science. The rumour on the street is that Morse attended a performance of Beethoven's Fifth Symphony on one of his trips to England. Idle speculation further has it that this performance so impressed him that the "dot dot dot dash" code he used for the letter "V" (which is also the Roman numeral for "five") was intended to emulate the symphony's opening sequence which goes "Da Da Da Daaa" . The telegraph quickly proliferated thanks to the relative simplicity of Morse's system. A problem soon arose, however, in that operators could only transmit around ten words a minute, which meant that they couldn't keep up with peoples' seemingly insatiable desire to send messages to each other. This was a classic example of a communications bottleneck . Thus, in 1857, only twenty years after the invention of the telegraph, Sir Charles Wheatstone introduced the first application of paper tapes as a medium for the preparation, storage, and transmission of data. Sir Charles' paper tape used two rows of holes to represent Morse's dots and dashes. The following illustration reflects the way I originally thought this worked, but someone once informed me that the actual scheme was somewhat different (I seem to recall that a dot used one hole on one side of the feed holes while a dash used two holes on either side of the feed holes, but I may be mistaken). Unfortunately, I misplaced the original message, so if anyone knows the real-world technique please let me know and I will update my diagram). Wheatstone's perforated paper tape Outgoing messages could be prepared off-line on paper tape and transmitted later. By 1858, a Morse paper tape transmitter could operate at 100 words a minute. Here's a YouTube Video showing the use of a Morse code system employing a two-channel paper tape mechanism. Unsuspectingly, Sir Charles had also provided the American public with a way to honour their heroes and generally have a jolly good time, because used paper tapes were to eventually become a key feature of so-called ticker-tape parades . The Printing Telegraph As was discussed in the previous topic, the first telegraph machines were invented in 1837 by Sir Charles Wheatstone in England and Samuel Finley Breese Morse in America. Morse's machine was eventually adopted as the standard because it was simpler, easier to construct, and more reliable. Morse's original machines kept a record of incoming messages using an electro-mechanically-controlled pencil that made marks on a moving strip of paper. The paper was driven by clockwork, while the lengths of the marks corresponded to the dots and dashes used in Morse Code. However, operators quickly realised that they could recognise the message by sound alone, so Morse's recording devices returned to the nether regions from whence they came. Throughout the rest of the 1800s there continued to be a strong interest in the idea of a printing telegraph. Much of the work towards realising this dream was based on the concept of a wheel with characters embossed around the periphery. The idea was to use the incoming telegraph signals to spin the wheel by fixed steps until the correct character faced the paper, and to then propel that character onto an inked tape located in front of the paper. There were a variety of techniques for controlling the wheel, such as a single pulse for 'A', two pulses for 'B', three for 'C', and so on, with the wheel returning to a home position after each character, but this technique was very slow in terms of words-per-minute. Later techniques used a five-bit code created by the French inventor Jean Maurice Émile Baudot in 1880, which soon became known as the Baudot Code .