tape store for telegraph. characters

250 Magnetic PHILlPS TECHNICAL REVIEW VOLUME 26... tape store for telegraph. characters H. van Kampen 621.318.24 :621.394 Teleprinters operating on the stop-start principle are used on a large scale in modern telegraph traffic. Switching functions in networks based on such teleprinters are now generally being automated. Either of two different principles may be adopted in this process, the choice depending on the nature of the network to be automated. In public telex networks, the connection between the originator and the receiver of the message is established before the exchange of information begins. In many non-public networks, however, a method has been chosen whereby messages are forwarded from centre to centre and, where necessary, recorded temporarily in a store until the circuit to the next centre is free. An article by P. Harkerna in the present issue [1] contains a description of automatic switching centres designed upon this principle and incorporating groups of storage units in which ferrite cores are used as store elements. One advantage of such stores which interests us here lies in the fact that the messages they contain can be read out two or more times for transmission to a number of different destinations. The article also points out that it is occasionally necessary to provide certain outgoing lines with individual stores. This may be necessary when a line has so much traffic to handle that.messages intended for it accumulate and threaten to overload the group of central stores. Provision of an individual store mayalso be desirable if the outgoing circuit employs a radio channel. On teleprinter circuits with radio channels, it is a common practice to use "TOR" (Telex Over Radio) equipment. This equipment is designed to detect mutilations which occur in the transmission of telegraph characters, and upon detection of a mutilation, ensures the retransmission of as many characters as are necessary to put the mutilation right. The number of such repetitions, especially when propagation conditions on the radio path are poor, may be high, so that the mean. signalling speed is considerably lower than the nominal one. The number of characters arriving at the channel input will then obviously be greater than the number leaving from the channel output. It is therefore necessary to insert at the input a recording device that can always accept messages from the stores in the central groups at nominal speed, and so ease the load on this Ir. H. van Kampen is 011 the staff of N. V. Philips' Telecommunicatie Industrie, Hilversum. group, while it can retransmit to the outgoing channel at a much lower average speed. In the cases mentioned, where individual stores are necessary, the messages need not be extracted from the store more than once. Also the storage capacity sometimes has to be so much greater than that of a central group store that ferrite core stores would be uneconomic and other solutions to the problem must be sought. ' Perforated paper tape has long been in use for storage purposes in telegraph centres. It satisfies the requirement as to storage capacity but has the disadvantage that it can only be used once. Where traffic is heavy, expenditure on tape is far from negligible. There is the further point that perforator and tape transmitter mechanisms are rather complicated and demand regular maintenance. Moreover the channel may be put out of operation if a reel of perforated tape runs out at an awkward moment. These considerations led to the development of the store described below, in which the storage function is performed by an endless magnetic tape of sufficient length to record 40 000 telegraph characters. This capacity is sufficient for all practical purposes. The disadvantages of paper tape have been wholly obviated by this solution. Fig. 1 shows how the 20-metre magnetic tape, on which 40 000 characters can be recorded, is housed in a cassette. Separate writing and reading heads, each with its own tape-feed mechanism, are fitted above the cassette. The design Investigation into the requirements that a magnetic tape store for telegraph characters must satisfy soon revealed that the design currently in use in popular instruments for recording music and speech would not do. In the first place, as is clear from what has been said above, the writing and reading functions must operate independently of each other and simultaneously. A magnetic head and a tape-feed mechanism must be available for each function. At the same time, separate tape-feed mechanisms would also make it possible to use different speeds for writing and reading. Teleprinters at present in use throughout the world do not all have the same speed of operation. In Europe and adjacent regions the CCITT has standardized a speed of 50 bauds. In the United States, however, speeds of 45.45 and 56.88 bauds have long been in

N.V PHILlPS' GlOflLAMPENFABRIEKfN 1965, No. 8/9 STORE FOR TELEGRAPH CHARACTERS 251 knows, consists of seven elements transmitted serially: a start element, five code elements and a stop element. Only the five code elements need be recorded on the tape. A separate track for each of them is available on the 1/2 in. wide tape. A series of alternating positively and negatively magnetized elements are written on a sixth track for control purposes. As already observed, the magnetic tape is moved forward one step for every telegraph character. The time taken to do so is considerably shorter than the time needed to transmit a complete character at the highest speed which occurs, namely 74.2 bauds. The tape-feed mechanism need therefore not be modified if the signalling speed is changed. Adaptation is entirely confined to the electrical control and in the present case can be done remotely. The distance the tape is moved per character is 0.5 mm, so that 20 metres of tape are needed for 40000 characters. This length of tape can be accommodated in a cassette of reasonable dimensions, as shown in fig. 1. From what has already been said, it will be obvious that the amount of tape needed is not affected by the signalling speed chosen. Fig. I. The 20 metres of magnetic tape needed to record 40000 teleprinter characters are housed in a cassette. Above the cassette are the writing and reading heads with their tape feed mechanism. use, while the considerably higher speed of 74.2 bauds is also at present in use. It is therefore possible, by choosing different writing and reading speeds, to use the storage device to couple networks in which different signalling speeds are used. The desirability of being able to work the store with TOR equipment makes it essential to be able to record characters on the magnetic tape one at a time. It must also be possible to control the starting and stopping of the tape from outside. In tape recorders for music and speech the tape is moved at uniform speed over the recording and playback head. Changes in the magnetization of the tape induce voltages in the coil fitted round the magnetic circuit of the head. These voltages are proportional to the frequency ofthe signal to be read out. At signalling speeds of 45.45 to 74.2 bauds the signal frequencies are so low that no useable output signal can be obtained. For these reasons a read-out method is used in which the output voltage depends solely on the value of the magnetic field at the point of read-out and not on the changes in the field along the tape. The output voltage therefore does not depend on the speed at which the tape moves over the head and is not affected by the fact that the tape does so step by step. A teleprinter character, as the reader probably Writing and reading As we have seen, the characters in the store dealt with here are not read by determining the variations of the magnetic field along the tape, but by measuring the absolute amount of magnetization at individual points. The method adopted is based on the principle put forward by Kilburn et al [21. We have departed from their procedure by impressing the magnetization perpendicular to, instead of along, the surface of the tape, so that distinct external fields and therefore individually measurable signals are obtained, even when there is a succession of elements with the same polarity A writing head and a reading head are shown diagrammatically in fig. 2. It should be realized that six such combinations are used. Fig. 3 gives an enlarged picture showing how the five tracks carrying code elements and the single track carrying control elements are arranged on the magnetic tape. In analogy with the procedure usually adopted for perforated paper tape, it is the third track which is reserved for control purposes. No further explanation of how the writing head works will be necessary. The magnetic circuit of the reading head is divided at one point in such a way as to form a closed magnetic sub-circuit. Round the arms ofthis sub-circuit are two windings W1 and W2. Winding [1] P. Harkema, Automatic telegraph exchanges with electronie stores, Philips tech. Rev. 26, 240-249, 1965 (No. 8/9). [2] T. Kilburn, G. R. Hoffman and P. Wolstenholme, Reading of magnetic records by reluctance variation, Proc..lEE 103, Suppl. No. 2, 333-336, 1956.

252 PHILIPS TECHNICAL REVIEW VOLUME 26 A Fig. 2. Schematic representation of the reading and writing heads. Writing head A, on the left, impresses magnetization perpendicularly on the surface of the tape T. The magnetic circuit of reading head B is divided at one point into two arms round which windings Wl and W2 are fitted. The function of this sub-circuit is described in the text. Fig. 3. Drawing showing how the five code elements of a teleprinter character are recorded on five tracks of the magnetic tape. A sixth track (track 3 on the tape) has a series of alternating positively and negatively magnetized elements impressed on it and is used for control purposes. Fig. 4. Circuit in which windings IVI and W2 of the reading head R are included. cp is the phase discriminator. Fig. 5. A part of reading head B shown in Fig. 2. The leads of windings IVI and W2 are terminated on a synthetic resin bonded paper panel, with the magnetic material at the centre. The head ends in a narrow point over which the magnetic tape runs. Below this point can be seen the division of the magnetic circuit into two arms. The reading heads are extremely flat because six of them have to be fitted together within the 1/2 inch width of the magnetic tape. Wl is arranged so that any current passing through it will set up circulating magnetization in the sub-circuit but not in the main circuit. Winding W2 is a test winding and insensitive to variations of circulating magnetization in the sub-circuit, but is sensitive to variations in the main circuit. An auxiliary a.c. current of frequency fpermanently flows through winding wi with an amplitude such that magnetic saturation periodically occurs in the subcircuit. As a result of this saturation, the magnetic resistance in the main circuit is always high except at the moments of polarity reversal of the current through wi. Only at those moments is it possible for the field of the tape to cause any perceptible magnetization in the main circuit. As there are two polarity reversals per cycle of the a.c. current through Wl, an alternating voltage with a frequency of 2f is generated in W2 Careful consideration will show that if the magnetic field reverses its direction when the tape moves, the phase of the a.c. voltage in Hl2 will change by 180. Use is made of this fact in the reading circuit shown in jig. 4. A generator sets up an a.c. voltage of frequency fwhich is applied to winding wi of the reading head R. An a.c. voltage with a frequency 2fis taken from Hl2 and applied to phase discriminator cp, via a band-pass filter for the suppression of unwanted frequency cornponents. The voltage used for comparison is a voltage of invariable phase and frequency 2f obtained from the generator freq uency by doubling. The phase discriminator is adjusted so as to deliver a positive output voltage for one direction of the field on the magnetic tape and a negative voltage for the other. Fig. 5 shows the construction ofthe part of a reading head on which windings Hll and W2 have been fitted. The magnetic circuit is closed by a yoke common to all six reading heads. Tape feed In each drive mechanism the tape is moved over a small steel roller. This roller is driven via gear-wheels by a motor, to be described below, and has a diameter such that the tape moves a distance of 3 mm for every complete revolution of the motor. As each character is allocated 0.5 mm of tape, the motor makes one sixth of a revolution for every character to be recorded. The roller is smooth and cannot be used to drive the magnetic tape directly. As the drawing on the left of jig. 6 shows, the tape is held against the metal drive

1965, No. 8/9 STORE FOR TELEGRAPH CHARACTERS 253 ---_e_- - --.- -- ----- -- - -------------- Fig. 6. Enlarged drawings of the magrietie-tape feed mechanism. The left-hand one shows how the tape runs between a small steel roller and one made of resilient rubber. As the steel roller is too smooth to drive the tape directly, the rubber roller is made wider than the tape. The drawing on the right shows how the steel roller drives the rubber roller on either side of the tape, which is thus driven by the centre section of the rubber roller. Fig. 7. The ferroxdure rotor (right) and one of the two stators of the drive motor. The stator consists of a disc-shaped coil between two plates, each of which has twelve teeth bent towards the stator. St! R St II st! R StIl St! R Sill Q Fig. 8. Schematic diagram of rotor R and two stators Str and Stn. In the unoperated condition a, a north pole of the rotor is positioned 'on the line connecting two south poles. In b the rotor is still in the same position, but the direction of the current in St! has been reversed. The rotor is subjected to an upward force and therefore moves to the position it occupies in c. roller by a roller of fairly resilient rubber. The rubber roller, being broader than the tape, projects beyond it on both sides, as illustrated in the right-hand drawing, and so presses against the steel roller as well. The friction between the rubber and steel rollers is sufficient for the latter to drive the former without slipping. The rubber roller in turn transports the magnetic tape, also without slip. The motor which provides the driving power has two identical stators and a ferroxdure rotor. The rotor and one of the stators are illustrated in jig. 7. Each stator consists of a disc-shaped coil between two softiron discs, each of which has twelve upturned teeth. When the coil is energized, twelve pairs of poles are set up at the stator teeth. The rotor is permanently magnetized in such a way that twelve pairs of poles also exist along its perimeter. As jig. 8 shows, the two stators st; and stn are offset relative to each other by half a pole pitch. When both stator coils are energized in a given direction, as for example in fig. 8a, the rotor will always position itself so that a north pole of the rotor lies on a line joining two south poles of the stators. If, as in fig. 8b, where the rotor still occupies the same position as in fig. 8a, the current through one of the stator coils is reversed, a north pole of the rotor will be repelled by the two stator north poles below it, and attracted by the two south poles above it. The direction in which the rotor will turn is therefore decided, and it will rotate half a pole pitch upwards to the position it occupies in fig. 8c. It can easily be shown that, if the current direction is switched in one stator coil and the other alternately, the rotor will advance regularly. The rotor has a total of 48 discrete positions and the motor has to make 8 steps to transport the tape over the length of one character. Operation as a whole Fig.9 is a schematic representation of the main parts of the store. On the left can be seen the receiving distributor A. This receives the telegraph characters whose elements arrive one after the other. The distributor is triggered by the arrival of the start element of a character. The five code elements are recorded in the same number of bistable (flip-flop) circuits. When the stop element is received, the contents of these five bistable circuits are transferred to five more in writing amplifiers C 1,2,4,5,6. Simultaneously, the correspon-

254 PHILlPS TECHNICAL REVIEW VOLUME 26 Fig. 9. Diagram illustrating the working relationship between the various parts of the store. A receiving distributor; B transmitting distributor; Cl to Co writing amplifiers; Dl to Do reading amplifiers; E control circuit; Fwriting head; G reading head; H storage loop; Ktape. in cassette; MI and M2 drive motors. ding bistable circuit' in amplifier C3 is changed over, so that the' character element recorded by this amplifier is given opposite polarity to that of the element which preceded it. The receiving distributor is is immediately ready for receipt of the next character. As the character elements are transferred to the writing amplifiers, a signal is sent to control circuit E, which switches on the drive motor MI associated with heads F and makes it rotate 8 steps. While the tape is advancing 0.5 mm during this process, the six elements are recorded on it at the same time. As we have already seen, the movement of the tape is so rapid that it is always completed before the receiving distributor has received all of the next character even if this character comes immediately after the preceding one. As long as the store has no more text for retransmission, the magnetic tape between the writing and reading head is taut. On becoming taut it operates a lever which opens a contact, causing drive motor M2 for reading head G to switch off. When the next message comes in, the motor of the writing head starts and part H of the magnetic tape will hang slack. The reading-head motor is therefore switched on again and the tape starts moving under the reading head. As we shall explain presently, there is no text on the length of tape between the writing and reading heads in the non-operated condition. Further, in this condition the elements of the control track written on this part of the tape all have the same polarity. No further change of polarity on this track occurs until the first character in the next incoming message is written. As soon as reading amplifier D3 detects a change of polarity-on the control track, the readinghead motor is once more stopped. Retransmission of the character which is under the reading heads then depends on whether the telegraph channel connected to the output of transmitting distributor B can accept the character or not. If the outgoing channel can take the character, it sends an appropriate signal to transmitting distributor B. Amplifiers DI,2,4,5,6 will consequently read the elements of the character simultaneously from the tape and transfer them at once to five bistable circuits in the transmitting distributor. The latter transmits the five code elements ofthe character sequentially, prefacing them with a start element and appending a stop element. At the moment transmission' begins, however, the motor at the reading head is started and the tape is moved forward until the next change of polarity on the control track is detected. Pending the next instruction from the circuit associated with the outgoing channel, the next character waits in readiness under the reading heads. If the message has been completely received, the r~ceiving distributor will detect no more start elements and the write motor will cease stepping. The motor at the reading end will continue moving the tape forward until it is taut. The lever referred to above now operates and switches off the read motor. As the length of tape between the writing and reading heads contains a part of the message which has not been'transmitted, the write motor is restarted by the control circuit, which allows it to run long enough to feed in all the tape necessary for complete retransmission of the "tail" of the message. The fact that the write motor is now running while no characters are coming in means that the polarity of the writing current through the third head does not change. Since a new message may start arriving while extra tape is being fed through, the circuit is arranged so that upon receipt of a start element the write motor immediately stops advancing extra tape and starts stepping again under control of the incoming characters. A final point which might be added is that when

1965, No. 8/9 STORE FOR TELEGRAPH CHARACTERS 255 I Fig. 10. Two complete stores are mounted on one frame. Left: two cassettes containing magnetic tape and, above them, the reading amplifiers in hermetically sealed containers. The other electronic equipment is mounted on panels which can be removed in groups for maintenance. Right: a supervision panel with lamps and jacks. the various character elements are recorded the magnetic layer is always saturated. It is therefore unnecessary to erase the recorded information after read-out. Arrangement of the equipment As jig. la shows, the cassettes illustrated in fig. 1 are mounted in pairs on a frame with the associated equipment. The two sets of reading amplifiers are mounted immediately above the cassettes in hermetically sealed containers. These are necessary because the amplifiers have to operate within very close tolerances. The writing and reading heads in the cassettes are magnetically screened to protect them from disturbing influences such as the earth's magnetic field. The remaining electronic equipment, is less sensitive, and is mounted on narrow strips of SRBP arranged vertically in the middle of the rack but combined in horizontal groups to form units. For maintenance each such unit can be withdrawn from the rack by means of the bar running along the front of it. A number of pilot lamps, test jacks, etc., are assembled on a supervision panel installed to the right of the rack. Summary. A teleprinter character store for use in automatic telegraph centres is described. A capacity of 40 000 characters has been attained by the use ofmagnetic tape as the storage medium. In view of the low frequencies used to transmit telegraph characters, a static method is employed in reading characters from the tape. The latter does not run at a uniform speed but is moved forward a step at a time. This is done by means of a stepping motor of very simple design. The store is suitable for the various speeds at which teleprinters are normally operated, the necessary matching being effected completely electrically, without any adjustment of the mechanism.