Patent GB-1140102-A (GB)
Title: Code communication system
IPC: H04L1/00
Filed: 1966-05-31
Abstract: 1,140,102. Selective signalling. GENERAL SIGNAL CORP. 31 May, 1966 [8 June, 1965], No. 24148/66. Heading G4H. In a digital communication system a central station can transmit selectively to any one of a plurality of field stations by frequency shifting a first carrier frequency up or down to represent marks or spaces respectively, and simultaneously the field stations can transmit, one at a time, to the central station by frequency shifting a second carrier frequency. Central station to field station.-Whenever the central station wishes to transmit to a field station, the address of the field station (10 bits) and the data to be transmitted (20 bits) together with an initial one bit, referred to a "tag" bit, are inserted into a 31 bit magnetic core shift register. Simultaneously a check counter is pre-set to 31 to block the entry of further data into the register. The 31 bits in the register are then shifted out serially, tag bit first, by clock pulses, to control a frequency shift transmitter. At the same time the check counter is counted down to allow more data to be entered into the register once it is empty. At each field station the transmitted data is received, appropriately demodulated and shifted into a 31 bit data shift-register by means of clock pulses derived by mixing received ones and zeros. The tag bit is also entered into a recognition shift register which is so constructed that the application of the following 30 bits will cause the tag bit to be shifted right through if the address which the recognition shift register is designed to recognise is received. Thus, at the called field station, providing it responds correctly, the tag bit will reach the end of both the data shift register and the recognition shift register simultaneously. Detection of this condition causes the data in the shift register to be transferred, in parallel, to a utilization device. At a non-called field station, the tag bit in the recognition shift register will, at some stage, be cleared instead of being shifted. The contents of the data shift register are not then transferred to the utilization device. The central station is so arranged that when it is not transmitting data, it transmits a series of zeros, so that each field station can always derive a clock-pulse train. Field station transmission.-The field stations are arranged to transmit serially, one at a time, the order of transmission being determined by roll call counters, one at each field station. At the beginning of a roll call cycle, that is a cycle in which each field station transmits once, the central station interrupts its own clock for seven bit periods. This interruption may occur at any time, even during the transmission of data from the central station to the field stations. The seven bit pause is detected by all the field stations, and causes their roll call counters to be set in accordance with the order in which the field stations are to transmit. Each field station is allotted 34 bit periods in which to transmit a 31 bit message which is of a similar form to that transmitted by the central station. The roll call counter of the first field station is set to 3 (to allow an initial pause), that of the second to 37, that of the third to 71 and so on. When the clock at the central station is restarted, every roll call counter is stepped backwards synchronously by clock pulses derived locally from signals received from the central station. When a roll call counter reaches zero its field station admits in parallel form data that is to be transmitted, into a shift register (this takes 1 bit period) and then transmits it serially, tag bit first, to the central station by means of an arrangement similar to the transmitting arrangement at the central station. Because the entry of data into the shift register occupies 1 bit, and because the field station starts to transmit at that time, the first bit it transmits, before the tag bit, is a zero. At the end of the transmission there is a 2-bit pause, during which the unshifted second carrier frequency is transmitted before the next field station begins. Central station reception.-At the central station the pause of seven bit periods, which initiates a roll call cycle, resets both a binary station counter, and a ring station counter. Data received from the first field station is entered into a shift-register, and the received tag bit is used to advance both station counters. Once the complete data word from the first field station has been fully entered into the shift register, the address contained in the shift register (i.e. the address of the field station from where the data came) is compared with the count in the binary station counter. If there is agreement (as there should be) the contents of the shift register are transferred in parallel to a row of a core matrix store selected by the ring station counter. A similar sequence of operations is repeated for each successive field station. Thus, the two station counters are advanced one step for each tag bit received and their counts should correspond to the address of the transmitting field station. One station counter is used to check that the correct field station is transmitting and the other to route received data to the appropriate row of the core matrix store. Should any field station not transmit at its appropriate time, this being detected by the absence of the zero which should precede the tag bit, the counters are advanced by a simulated tag bit and the core matrix is not up dated in respect of that field station. Also a lamp is lit to indicate that a field station is faulty, and the lamp will remain lit until that field station does transmit. Means can also be provided whereby if the field station does not transmit before a certain time, a second warning device is activated.