Patent GB-1398647-A (GB)
Title: Sequential tone signalling system
IPC: H04L1/00
Filed: 1973-09-14
Abstract: 1398647 Radio signalling MOTOROLA Inc 14 Sept 1973 [4 Oct 1972] 43302/73 Heading H4L The invention relates to a system for transmitting sequential tones representing messages to or from for example remote motor vehicles. Base station transmitter.-The signal to be transmitted, consisting of a vehicle address and other data is set up on keyboard 10 and stored at 14 in locations determined by depressed key detector 16 and memory location counter 18. The signal format, Fig. 2, is a sequence of tones of predetermined duration in which the first tone 51 alerts a particular system that a message is forthcoming. This is followed by data tones 52, 53, representing a particular message, followed by address tones 55, 56, 57 for a particular mobile recevier, It is arranged that two successive tones of the same frequency cannot be transmitted, to avoid false triggering of a receiver, a repeat tone R being sent in place of the actual repeat tone. As described, the tone 51 can be any of the tones 0 to 6, tone 52 can have value 7, 8 or 9, tone 53 can have any value not appearing at 52, tone 54 is always the repeat tone and serves to separate the last data tone from the first address tone 55 and tones 55, 56, 57 can take any of the values, logic circuitry being included to insert a repeat tone where necessary. The system tones 51 are preprogrammed in the memory 36. After the message has been entered into store 14, an end-of-message signal via sensor 26 enables gate 20 so that the tones from stores 14 and 36 are supplied sequentially to repeat tone comparator 30, which inserts the repeat tone as required. The gate 20 is controlled by clock 24 via an 8-step counter 22. The output of comparator 30 via binary-todecimal converter 36 causes oscillator 34 to vary its frequency in accordance with the tone sequence and the resulting signals are supplied to transmitter 35 which is energized by end-of-message sensor 26 via control circuit 28. After transmission the counter 22 provides a signal to stop the transmission. When signals received at the base station are being acknowledged the tone sequence is supplied to the transmitter from memory 36 and from a decoder, Fig. 3, via a selector 38 which is controlled by the decoder and the counter 22 via AND gate 21. Base station receiver.-A sequence of tones from a mobile station received at 61 is supplied to a bank of eleven filters 60 and a programmable filter 62. Filter bank 60 provides a signal representing the first tone of the sequence, which is converted to binary form at 64 stored at 66 and also causes filter 62 to be tuned to the frequency of the last tone received. The signal from converter 64 is also supplied to decoder control 68 to make a counter 76 respond to detected signals from filter bank 60 and provide stepping signals for memory 66. The output of converter 54 is also supplied to a circuit 70 to check that the received signal is of the correct format and to terminate the decoding if the signal is incorrect. Filter 62 passes a signal to the signal checking circuit 70 if the duration of a received tone exceeds a predetermined time interval. The output of filter bank 60 is also supplied to circuit 70 to determine whether another tone has been detected too soon and clock control 74 and time out clock 72 supply a signal to circuit 70 if a new tone is not detected more than a predetermined time interval after the previous tone. After the correct sequence has been received it is transferred from store 66 to display memory 67 which causes display 80 to indicate which message has been received and simultaneously the message is supplied to selector 38, Fig. 1, for retransmission to the originating station for checking. Counter 76 provides a reset signal for circuit 70 and a pulse to start transmitter 35. If it is not required to acknowledge a message, memory 67 provides a signal to inhibit circuit 78. Mobile station transmitting, Fig. 4.-Switches 104, 106 are in the position shown so that programmable active filter 102 oscillates to supply tone signals to transmitter 103, the frequencies being controlled by a matrix of programming resistors 108 under the control of a shift register 114. The values of the resistors switched in are determined by data sources 110, 112, providing message and address signals. At the end of the tone sequence, the shift register 114 provides a signal via control circuit 116 to switch off the transmitter 103 and open switch 104. A push button is operated to start a transmission whereby circuit 126 provides an input for AND gate 120 and a channel monitor 124 provides a second input when no signal is present on the channel. The remaining input is supplied by a random clock 122 which prevents two stations from transmitting at the same time. After the message has been received at the base station it is retransmitted to the mobile unit where it is compared with the last transmitted message stored at 110 and 112 and if it corresponds, the unit returns to its rest state. If there is no acknowledgement within a predetermined period, circuit 126 via AND gate 120 causes the signal to be retransmitted and this may be repeated for a predetermined number of times or until the message is acknowledged. If it is not acknowledged a light is energized to warn the operator. Acknowledgement of the message turns the light off and initiation circuit 126 is reset by circuit 128. Mobile station receiving, Fig. 5.-Switch 104 is open and switch 106 closed so that received tones are supplied to the programmable filter 102 which is controlled as before by resistor matrix 108 and shift register 114. In the rest state a programmed message, e.g. a request for a call is stored at 112 and the receiver responds only to that call. If the received frequency corresponds to the setting at 102, the tone is supplied to a detector 130 which causes shift register 114 to change the frequency of the filter 102 in accordance with the programme. The detector also supplies a signal to energy dump timer 132 which dissipates the energy stored in filter 102. This process is repeated until the complete signal has been received and a display 136 is operated. An interconnection may be provided (shown dotted) between control circuit 116 and resistor matrix 108 to enable the circuit 108 to be programmed with the rest state message. The dotted connection between circuit 116 and reset 128 inhibits operation of the reset when the rest state message is programmed into circuit 108.