.--------------------. r I ' An introduction to Creed teleprinters and punched paper tape equipment An introduction to Creed teleprinters and punched paper tape equipment CONTENTS Introduction What is a teleprinter?- The teleprinter code 2 Start/stop principle- Telegraph signals 3 Telegraph speed- Outline description of a teleprinter- Stages of transmission 4 Keyboard - Receiver 5 Punched tape technique- Kinds of punched tape- Methods of coding information on punched tape 6 Applications to digital computers­ Input preparation- Manual tape preparation- Tape editing 10 Output recording and printing­ Model Seventy-five interpreter set­ Model Seventy-five 5-wire reproducer set 11 The equipment- A short introduction to each of the major Creed machines 12 Glossary of terms 16 INTRODUCTION Creed teleprinters and punched paper tape equipment were originally devel­ oped for use in the telegraph com­ munication field and they have been increasingly used ever since in telegraph systems all over the world. During recent years, however, their use has been extended beyond this traditional field to a rapidly growing number of non-telegraphic applica­ tions such as the provision of input and output facilities for digital com­ puters and the automation of existing systems such as punched card accounting and mechanised addressing. As a result of this sudden increase in the number of applications that are being found for teleprinters and punched tape, considerable interest has been shown in this equipment by engineers and others who wish to discover whether it can be applied to their own special problems. This bulletin has been written princi­ pally for such readers and others who need to know of the basic principles used in teleprinter communications systems. It contains a brief, non­ technical introduction to the general principles underlying the operation of Creed teleprinters and punched tape equipment, and includes a brief description of each of the various machines available. It must be emphasised that this bulletin does not describe all the non­ telegraphic applications that have been made to date of this equipment, nor does it contain any engineering details of circuits or machines. Full technical details and free advice on how any Creed equipment can be used in telegraph or data processing systems is available on request. Any system for transmitting and re­ nsmission of 1. Run-Out Key. The deprdssion of ceiving messages electrically over a paper on the this key results in the last signal sent distance in the form of coded signals to be fed up to being repeatedly transmitted for as is called a telegraph system. rec~ived message long as the key remains depressed. If the messages are automatically is printo A!-Tn..-.....m;;~ e 'Line Feed' 2. 'Here is' Key. The depression of printed by the receiving apparatus, the function is not, of course, used). this key causes the key oard trans­ system i;> then r'eferred to as a printing 2. Carriage Return. This function mitter to send automatically the calling telegraph system. causes the carriage on the receiving station's identifica ron code signal to The teleprinter is the basic piece of teleprinter to return to the beginning the called station. equipment of the Creed printing tele­ of its travel so that the printing starts graph system. It consists of two parts: again at the beginning of a line. THE TELEPRINTER a keyboard transmitter and a receiver. (Again, with ta e print:ng, this is CODE not used). The l<eyboard transmitter consists of a The code used for the transmission of l<eyboard, similar in appearance and 3. Who-are· You? This function intelligence from one teleprinter to layout to a typewriter keyboard, for causes the distant teleprinter key­ another is a 5-unit, 2-element (binary) 5 originating the message, and a board transmitter to send back auto­ code whic allows a total of 2 , i.e. transmitter for converting the opera­ matically to the calling teleprinter, thirty-two combinations. The method tor's key depressions into suitably where it is printed, a series of charac­ of allocating these co inations to coded electrical signals which it ters and/or numerals informing the the various items of intelligence has transmits to the line or other medium calling operator of the identity of the been the subjec of various confer­ of transmission. distant station. This assures the ences held by the CCITT (Inter­ calling operator: national Telegraph and Telephone The receiver is a device for registering Consultative Committee), a body the coded signals that are received a) that he is connected to the right station; which represents most of the main from the distant transm· ter and telegraph interests in the world and converting them into a printed b) that the called teleprinter is operat­ exists to promote, among other message on a page or tape. ing even if unattended; and things, the growth of common prac­ The intelligence transmitted consists c) if operated at the end of a message, tices in telegraphy. The method of of characters, numerals, signs and that the whole message has been code allocation indicated in Fig. 4 is functions. The most commonly used received. the CCITT International Code No. 2, items of intelligence are shown in the 4. Bell. The transmission of this which is the one at present in general three typical keyboard layouts function causes a bell to ring (or pro­ use. illustrated in figs. 1 - 3. duces some other warning indication) It will be noticed that the two kinds of The characters in all three layouts are at the called station to attract the elements of the code are called 'mark' the twenty-six letters of the English operator's attention. and 'space' elements. These terms alphabet. While this is the most 5. Space. This is similar to the were derived from telegraph systems common arrangement, layouts for normal spacing tunction on a type­ employing the Morse Code, where the other alphabets with more or less than writer. Its transmission results in the dots and dashes were referred to as twenty-six letters are available. carriage on the receiving teleprinter 'marks' and the spaces between the Similarly, althoud'h the numerals in feeding along one space without dots and dashes simply as 'spaces'. these layouts are confined to 0 and printing. In connection with the 5-unit code these meanings are, of course, irrele­ 1 - 9, these may be augmented by 6. Letters and fractions. vant, but the terms have been adopted 1. Figures. These functions have a as convenient labels for distinguishing Signs such as'%','@' and'?', may purpose which will be explained in the between the two kinds of elements of also be varied to suit special require­ next section. the code. A 'mark' element may be ments, such as data processing In the keyboard layouts in Figs. 1 - 3, defined, therefore, in terms of the instructions or weather charts. two keys ('Run Out' and 'Here is') International Code as any element of Seven functions are controlled from have not so tar been referred the code represented by a solid dot in the keyboard. They are Line Feed, to, since they are not strictly speaking Fig. 3; similarly, a 'space' element is Carriage Return, Who-are-You?, Bell, items of intelligence, i.e. no code Space, Letters and Figures. The signal is allocated to them. Their purpose of these functions is as purpose is as follows: follows: 2 TELEGRAPH SPEED OUTLINE DESCRIPTION STAGES OF The unit of telegraph speed is called OF A TELEPRINTER TRANSMISSION the 'baud', after the famous French The basic elements of a simple point­ There are a number of well-defined telegraph inventor Baudot. It is equal to-point teleprinter system are stages and processes between the to the number of shortest telegraph represented in Fig. 5. depression of a key and the impres­ signals, i.e. units, per second. The transmitter consists essentially of sion of the selected character on the The present international standard is a metal tongue T which, when a key is paper in the distant printer. These are 50 bauds, which makes the unit equal depressed, is caused to move between shown schematically in Fig. 7 which, to 20 milliseconds. With 7!-unit trans­ two contacts M (mark) and S (space) for clarity, represents the stages for mission, the 'Start' signal and the five in a manner determined by the code the transmission of a particular letter code signals are, therefore, each 20 combination of the key depressed. -A, for a double-current teleprinter. milliseconds in length while the 'Stop' Voltages of opposite polarity are These general principles apply to all signal is 30 milliseconds, and the connected to the two contacts and the conventional teieprinters now in transmission time for a complete 7!­ tongue is connected to the line. production, but the means of achieving unit transmission is 150 milliseconds, the same end result vary quite The basic element of the receiver considerably from model to model. equal to 6·6 characters per second. consists merely of an electromagnet, Another way of measuring telegraph one side of which is connected to line For this reason it is not possible to speed, which is very useful, is in words and the other side to earth. describe here in detail the sequence of operations between the depression of per minute. In order to obtain a word­ Thus, when a key is depressed, a measure, five letters and a space are a key and the printing of a character on sequence of square-wave pulses the distant teleprinter. The following taken to be the average length of a (ignoring distortions caused by the stages, however, apply to all current word in English, i.e.