Patent GB-926321-A (GB)
Title: Data transmission system
IPC: H04M11/06
Filed: 1959-10-22
Abstract: 926,321. Electric selective signalling. WESTERN ELECTRIC CO. Inc. Oct. 22, 1959 [Nov. 3, 1958; Jan. 5, 1959], No. 35819/59. Class 40 (1). [Also in Group XIX] For the transmission of data over a line, each digit is represented by a combination of two out of six possible frequencies and the digits are transmitted serially. The frequencies are produced at the transmitter by oscillators in circuit 15, Fig. 1, and the data is entered from keyboard 16 and from a card reader 17. Control signals are sent to the receiver via a further keyboard 20. Power to the oscillators is supplied from battery 18 through switch 19 and the output signals are applied to telephone lines by a coil 23. Transmitter.-The keyboards 16 and 20 are mounted on the upper surface of unit 21, Fig. 3, and the card reader 17 takes the form of a tray 25 into which the card is inserted and moved relative to sensing means in the unit 21. The telephone set 12 is connected to the transmission line through the unit 22, the hook switch 19 serving to apply the power to the oscillator unit 15 when the receiver is off the cradle. This is shown in Fig. 2 together with the keyboards, card reader and coil 23. Unit 15 consists of a number of conventional oscillators 60-65 having output terminals C-H. These are connected in pairs to the switches 1-9 of keyboard 16 and also to the switches of the control keys 20. When a key is pressed the blades 69, 70 spread to engage contacts 71, 72, thereby connecting two of the terminals C-H to the coil 23 so that two of the frequencies from the oscillator unit 15 are transmitted over the line. The card reader has a tray 25, Fig. 4, into which the card is placed and located by guides. The tray is pulled out of the body 21 by hand and returns by spring action its speed being controlled by a governor 82 and the data is sensed column-by-column during this movement. The card when in position in the tray lies on a series of printed contact bars 91, 91<SP>1</SP> alternately connected in common to bars 92, 92<SP>1</SP>. Sensing is effected by pairs of feelers each pair being able to pass through a data hole to engage the underlying pair of contact bars 91, 91<SP>1</SP>. The common bars 92, 92<SP>1</SP> are connected through contacts 94 to the output lines and each pair of feelers is connected to a characteristic pair of oscillator outputs C-H so that when a hole is sensed the corresponding pair of frequencies are transmitted over the line. Instead of having holes the record card may have embossments to represent the data which are sensed as shown in Fig. 10 by the central one of a group of three fingers 113, 114, 115. Transverse bars 116, 117 carried by the central finger 114, insulated from each other, are connected to two of the oscillators C-H. When the finger 114 engages an embossment the bars 116, 117 contact the fingers 113, 115 and the corresponding frequencies are applied to the transmission line. In the form of Fig. 11, a single feeler 120 can detect embossments 124 or holes 126, the upper contacts being closed for the one and the lower contacts for the other. The pairs of contacts are connected together and a frequency may be sent for each pair; that is three altogether. A constant end of message signal may be transmitted from an area of the tray beyond the record card, this area being covered with a mask having openings at appropriate points to allow the fingers to engage the contact bars. Receiver.-In the receiver, Fig. 1, the data is received in the multi-frequency code and is translated to its original form. A line switch 30 connects the transmission line to a hybrid circuit which routes signals into or out of the receiver. If the power fails, line relays 32 operate to disconnect the receiver and notify the transmitter. The limiter 33 passes a signal of the proper amplitude to the signal detector 34 and to a " signal present " circuit 38. Detector 34 consists of a series of sharply tuned filters, the output of which pass to a code circuit 35. This consists of six relays, two of which are energized by the received signal. These relays control a further six relays in a translation unit 36, the relay contacts being connected in a network to produce an output on one of a number of lines which pass to a utilization device such as a record card punch. Checks.-Confusion caused by the overlapping reception of two signals is prevented by the signal present circuit 38 which produces an output when a signal is detected. This output is applied to a bias circuit 39 controlling detector circuit 34. The circuit 34 is rendered inoperative as the result of an output from the signal present circuit 38 so that a subsequent signal is ineffective while a first signal is still present. The output of signal present circuit 38 is also applied to the code circuit 35 to enable it to receive the output from the detector 34. This prevents operation of the code circuit 35 by spurious pulses. The interlock circuit 40 responds to a signal from utilization device 37 that it has effected registration of a signal to disconnect the signal from translate circuit 36 to the utilization device. This prevents the same digit being registered as two successive digits in the utilization device. The interlock is cleared when the output from signal present circuit 38 ceases, the circuit 36 being then reconnected with the utilization device. A lamp 41 controlled by the interlock 40 flashes to indicate that signals are being received. A code check circuit 43 consisting of marginal relays checks that the signal received comprises the prescribed number of frequencies. One of the marginal relays responds if more than two of the relays in code circuit 35 is picked and the other relay fails to respond if less than two are picked. Each record card is provided with a check digit calculated from the data digits. This check number is transmitted with the data and is applied to the mathematical check unit 44 where the calculation is repeated and the result compared with the transmitted check digit. The outputs from check circuits 43 and 44 pass to an error memory 45 which consists of a relay. At the end of an item transmission the transmitting operator presses button 46 to send the " Item Register " signal. This causes the translate circuit 36 to pass an output to the item register circuit 47 which clears the utilization circuit 37 for the next item and sends a check signal to a tone release circuit 48 to indicate that a complete item of data has been transmitted. Circuit 48 interrogates the error memory 45. If no error has been detected, the circuit 48 applies a steady signal from oscillator 49 through hybrid circuit 31 to the telephone line. If an error signal is stored in the memory 45 the circuit 48 actuates a pulsing circuit 50 to apply a pulsating signal to the line to indicate by an audible tone on the telephone set at the transmitter than an error has been detected. The " message end " key 51 at the transmitter causes transmission of a characteristic combination of frequencies which, at the receiver, cause the translate circuit 36 to pass an output to the message end circuit 52. This performs the same functions as the item register 47 and further lights a lamp 53 to indicate to the operator that the complete message has been received and the receiver may be disconnected from the line. If the transmitting operator realises that he has made an error in his transmission, he presses key 54 which causes the translate circuit 36 to send a clearing signal to the utilization unit 37 so that the item can be re-transmitted. To call the operator at the receiver, an " Operator key " 55 is pressed. This causes circuit 36 to light a lamp 56 and a buzzer may operate as well. A message start button 57 at the transmitter may be provided, the effect of which is to switch the receiver into operation. This protects it from spurious signals. Modification.-In a modification the double line output circuit of Fig. 2 is replaced by a single line circuit which simplifies connection to the telephone line. These are two separate oscillator circuits each able to produce one of a number of frequencies. For each signal one of the outputs of each oscillator is connected to the transmission line. A modified card reader has a tray which can accommodate two record cards and there are two sets of sensing fingers. One of these cards is permanently in position and provides data identifying the transmitting station, the other card is for item data and is changed for each transmission. Instead of lying flat as in Fig. 3, the tray may be inclined to facilitate insertion of the card. The cards rest on a surface provided with embossments 360, Fig. 23, at all possible perforation positions. This allows the feeler 362 to begin its downward movement before arriving at the perforation position and increasing the time for which its switch is closed. Operation of the card reader during the manual movement of the tray is prevented by a switch opened on the reverse movement by a rolling wedge 366, Fig. 26. Specification 873,189 is referred to.