标签: baudot

  The two-channel paper tape technique pioneered by Sir Charles Wheatstone was subsequently extended to five channels so as to handle the Baudot Code as illustrated below.   Paper tape showing the five-bit Baudot Code   Using a five-bit code, it is possible to obtain 2^5 = 32 different combinations of holes and blanks (no holes). In the case of the Baudot Code, twenty-six of these combinations were used for letters of the alphabet, leaving eight spare combinations for an idle code, a space code, a letter-shift code, and so on.   The problem was that there weren't enough spare combinations left over to represent the numbers '0' through '9' or any punctuation characters. In order to solve this dilemma, the letter-shift code was used to emulate the shift key on a typewriter by instructing the receiver that any subsequent codes were to be treated as uppercase characters (in this context, "uppercase" was used to refer to numbers, punctuation, and special symbols). A second letter-shift code could subsequently be used to return the receiver to the alphabetical character set.   The five holes and blanks for each character were transmitted as a sequence of pulses and gaps, and decoded and printed at the receiving end by a variety of different techniques. Note the special characters such as BELL, which actually rang a bell on the receiver to alert the operator that something interesting was about to happen, or was in the process of happening, or had just happened.   The early systems required the operator to use a keypad with five separate keys and to simultaneously push whichever keys were required to form a character. Later systems were based on a Typewriter-style Keyboard, whereby each typewriter key activated the five transmitting keys (or a paper tape punch) to establish the correct pattern. Unfortunately, none of these systems were tremendously robust or reliable, and they all suffered from major problems in synchronising the transmitter and the receiver such that both knew who was doing what and when they were doing it.   The original Baudot code became known as the International Telegraph Code No. 1 . Sometime around 1900, another 5bit code called the Murray Code was invented. The Murray Code eventually displaced the Baudot Code and became known as the International Telegraph Code No. 2 . Unfortunately, everyone was hopelessly confused by this time – Murray's name sank into obscurity, while Baudot's name became associated with almost every 5bit code on the face of the planet, including the International Telegraph Code No. 2   Storing computer programs and data on paper tape In a similar manner to Sir Charles' telegraph tape, the designers of the early computers realised that they could record their data on a paper tape by punching rows of holes across the width of the tape. The pattern of the holes in each data row represented a single data value or character. The individual hole positions forming the data rows were referred to as channels or tracks , and the number of different characters that could be represented by each row depended on the number of channels forming the rows.   The original computer tapes had five channels, so each data row could represent one of thirty-two different characters. However, as users began to demand more complex character sets, including the ability to use both uppercase characters ('A',''B', 'C', ...) and their lowercase equivalents ('a', 'b', 'c', ...), the number of channels rapidly increased, first to six and later to eight as illustrated below.   Computer paper tape with eight channels The above illustration represents one of the more popular IBM standards – a one-inch wide tape supporting eight channels (numbered from 0 to 7) with 0.1 inch between the punched holes. The first paper tape readers accessed the data by means of springy wires (one per channel), which could make electrical connections to conducting plates under the tape wherever a hole was present. These readers were relatively slow and could only operate at around fifty characters per second. Later models used opto-electronic techniques, in which a light source was placed on one side of the tape and optical cells located on the other side were used to detect the light and thereby recognise the presence or absence of any holes.   In the original slower-speed readers, the small sprocket holes running along the length of the tape between channels 2 and 3 were engaged by a toothed wheel to advance the tape. The higher-speed opto-electronic models used rubber rollers to drive the tape, but the sprocket holes remained, because light passing through them could be detected and used to generate synchronisation pulses.   On the off-chance that you were wondering, the reason the sprocket holes were located off-centre between channels 2 and 3 (as opposed to being centred between channels 3 and 4) was to enable the operator to know which side of the tape was which. Of course, it was still necessary to be able to differentiate between the two ends of the tape, so the operators used scissors to shape the front-end into a triangular point, thereby indicating that this was the end to be stuck into the tape reader.   Teleprinters and batch modes These days we're used to working with computers in what we call an interactive mode ; that is, you issue a command via the keyboard or the mouse and the computer responds almost instantly. Additionally, during the course of the program your computer may request further information from you, and both of you can interact together in real time.   This level of interaction was almost undreamed of in the not-so-distant past. Many of the first computers were controlled using a bank of switches known as a Switch Panel. Although controlling a computer by this means was interactive in its own way, it's not what we would regard as being interactive today. Furthermore, there was only one switch panel, but it could take many hours to enter a program byte by painful byte. Thus, if the switch panel had been the only way to enter programs, it would have been a pitiful sight to see professional programmers manhandling each other in a desperate attempt to claw their way to the panel.   In reality, the computer's time was far too valuable to have it sitting around twiddling its metaphorical thumbs while operators entered programs via the switch panel. Due to their extremely high price-tags, the only way for early computers to be cost effective was for them to be performing calculations and processing data twenty-four hours a day. Thus, although it had other system-management functions, the switch panel was mainly employed to enter simple boot-strap routines when power was first applied to the system. These routines were then used to load more complex programs from external devices such as paper tape readers.   The end result was that, in order to support the most efficient use of computing resources, it was necessary for programs to be created off-line. This makes a lot of sense when you realise that it might take the computer only a few seconds to run a program that had required many hours to enter. One technique for creating a program off-line was by means of a Teleprinter , which looked something like a typewriter on a stand with a large roll of paper feeding through it.   In fact, a teleprinter was essentially an electromechanical typewriter with a communications capability. In addition to the roll of paper (which was used by the operator to check what he or she had actually typed), the teleprinter could also contain other devices such as a paper tape reader/writer. (Teleprinters were often referred to as teletype machines or teletypes . However, this was a brand name (much as "Hoover" is for vacuum cleaners) for a series of teleprinters manufactured by International Telephone and Telegraph (ITT).   Here is a YouTube Video showing a Teletype Model 33 ASR equipped with an eight-channel paper tape mechanism.