GB1333951A - Electrical supervisory system - Google Patents

Abstract

1333951 Data transmission STANDARD TELEPHONES & CABLES Ltd 8 March 1971 6238/71 Heading H4P [Also in Division G4] An electrical supervisory system includes a number of stations between which signals can be transmitted using two channels, the bit rate of the signals to be dealt with by both channels being the same, the signals being conveyed on the channels by modulation on rectangular waves whose frequency is a multiple of that of the signal bit rate, the first half-cycle of the rectangular wave having a preselected relation with the signification of the signal, and the rectangular waves used as signal carriers on the two channels having different frequencies. The two channels use the same physical link in a microwave repeater system linking one or more control stations with a plurality of outstations, and are an interrogation and control channel from a control station to the outstations and a reply channel by which an outstation can send indications to the control station in response to an interrogation. These two channels use respectively n (at least 3, e.g. 7) and n/2 cycles of the respective rectangular wave to fill each bit period, which is the same in both cases, the bit value being indicated by the phase of this group of n or n/2 cycles, this being achieved by supplying to the 1 input of a D-type flip-flop the EXCL-OR of its 0 output and the bit value, with a bit rate clock applied to the common input of the flip-flop, the 1 output of the flipflop being supplied with the rectangular carrier to an EXCL-OR gate which feeds the line via a NAND gate and a band-pass filter. The NAND gate is used to inhibit the output if a signal is detected on the interrogation channel, i.e. due to another control station, so given differently-timed starts by different control stations only one control station will use the channel at a time. One " master " control station may periodically interrogate the others to see if they have control signals to send, and if they have, the " master " control station relays them on the interrogation channel. As a modification, the outstations may monitor the reply channel for control signals. A receiver involves carrier regeneration, the regenerated carrier and the line signal being phase-compared throughout the bit period using a ring modulator, doubly-clamped integrator and limiter, the coded data from the limiter being fed with the regenerated carrier to a sync unit (using counters and phase comparison) which produces a bit rate clock used in the retiming and decoding of the limiter output, this retiming and decoding being done in a circuit using two D-type flipflops and an EXCL-OR gate. The conditions at outstations for control/indication may involve circuit breakers in a power supply network or burglar alarms.