Patent GB-1366398-A (GB)
Title: Signalling method and apparatus
IPC: H04L5/14
Filed: 1972-02-21
Abstract: 1366398 Duplex signalling arrangement INTERNATIONAL BUSINESS MACHINES CORP 21 Feb 1972 [23 March 1971] 7855/72 Headings H4R and H4K Information is simultaneously transmitted between two stations over a two wire channel by utilizing signals of different polarity in one direction and by presenting different impedances to the channel for signals in the other direction. The first signals may be in binary format wherein a long interval between changes of polarity represents a "1" bit whereas a short interval represents a "0" bit. The second signals are derived by inserting different resistors across the line. General arrangement is shown in Fig. 1 where polarity reversals of the line wires L1, L2 is effected in a central unit 1 by appropriate manipulation of change-over switch 4, 10, 9. The line current traverses a selected one of resistors 16-19 in a terminal unit 2 but the current polarity is ineffective on a receiver in the central unit viz. meter 3, due to the connections of the latter to the change-over switch contacts. The receiver 13 in the terminal may respond to the incoming binary signals so as to control the resistance selector switch 14 in accordance with the value of the signals. The values may be indicative of: ready to receive, request transmission, busy, and error. An electronic central station is shown in Fig. 2. Terminals 32, 33 are pulsed alternately, each for a period proportional to the binary signal to be sent, so as to enable transistors 24, 25 respectively, and thereby to earth line wire L2, L1 respectively. Positive battery 5 is thus applied to the other line wire L1, L2 via resistor 29, 28 respectively. The arrangement detects the value of line current as determined by the selected resistor 16-19 with the aid of transistor 21 and 23 or 22 which form the diagonal of a bridge consisting of resistors 26, 30, 18 (as shown) and either 29 or 28. An indication of the current value is derived at terminal 34. A terminal station is depicted in Fig. 4. Incoming polarity changes on wires L1, L2 are applied to delay circuits 44, 47 so that if they are maintained long enough i.e. a "1" bit AND gate 45 or 46 will be enabled to open gates 49 and thus permit operation of a magnet. Each polarity reversal steps ring counter 48 so that the magnets can only be operated in sequence whereby synchronous operation of the central and terminal units is ensured. The impedance transmitter 72 includes a resistor 53 which may be connected across the line wires alone or in shunt with 54 or 67 in dependence on the conductivity of transistors 55, 60, 61 and 66. When 60 and 61 are both conductive the resistor has the value zero (in fact the internal impedance of line terminating circuits 40 and 41). The latter circuits include a diode 81 for ensuring that a positive polarity signal applied from the other line wire will always reach the line wire to which they are connected. They incorporate a pair of transistors (Fig. 5, not shown) connected to a winding of a transformer such that if the line wire is positive the transistors conduct and so cause the reflected impedance through the transformer to change from a high to a low value. As a result the output of an oscillator is effective through diode pairs (one pair for each half cycle of oscillation) to disable a transistor and thus repeat a positive voltage signal via terminal C to receiver 73. The original positive voltage signal applied to terminal B (and thence to the signalling resistors) via the first mentioned transistor pair. The drive circuits 42, 43 (Fig. 6, not shown), which determine the value of the resistor connected across the line wires, each consist of a pair of AND gates that are enabled alternately by half cycles from an oscillator if the inputs 1 are energized. A pulsating signal is thus produced across a transformer winding. The output from a secondary thereof is full-wave rectified so as to control a transistor which produces enabling signals for the transistors 55, 60, 61, 66.