Patent GB-1483760-A (GB)
Title: Multiplexed data transmission monitoring systems
IPC: H04L5/18
Filed: 1974-07-31
Abstract: 1483760 Multiplexing JOHNSON SERVICE CO 31 July 1974 [28 Nov 1973] 33849/74 Heading H4M [Also in Division G4] A system for multiplexing and transmitting information from a series of points during at least some of successive time frames comprises a number of monitoring at test points sampled in succession and the result transmitted, preceeded by a start pulse, to a remote location where the sequence is temporarily stored and an alarm circuit energized in response to change in. condition. The time frames are defined by a timing system which controls readout of information bits preceded by a start pulse, which enables a timing oscillator on reception. Each test point, e.g. in an air conditioning unit comprises a relay (K1-K7), Fig. 3 (not shown), closing, on "off normal" condition respective contacts (K1A-K7A) of which are energized cyclically by a transistor multivibrator (Q1, Q2) through counter (83) and commoned through gate (23), which operates a relay (25) having contacts shunting lines (105, 106). In operation, each active time frame scan (alternate frames may be inactive) comprises a 0 start pulse from line (91) and as many 1 pulses in sequence as contacts (K1A-K7A) are closed. At the receiver.-Through an LED isolating circuit 41 a start pulse bi-stable sensor 127 is set thereby enabling a double frequency clock oscillator including transistors Q5, Q6 providing pulses to a counter 44 and shift pulses to a 7 stage temporary register 120a-120g, the start pulse passing out of stage 120g at the end of each active frame. Shift pulses are at half clock rate and occur at bit centres. The content of register 120 is then passed parallelly into another register 151-157 stages of which are connected through isolating diodes D5 to an intermittent audio alarm device 51 through gates 160, 162. A manual reset push 164 is provided. A "permanent" register 171-177 also stores information pulses and has individual alarm circuits 59a, 60a &c.  A supervisory circuit, incorporated in the receiver, provides indication of line open or short circuit. A secondary counter 63 operates parallelly with counter 44 and if a start pulse appears within a period determined by counter 63 from frame start time, which should be every 18 time slots, then the line is assumed to be normal. Otherwise a gate 213 is enabled and an alarm energized. An NAND gate 229 may also be enabled by outputs of counter 44 during a start pulse to prevent receiver 40 responding to short breaks in transmission line less than 50% of one transmission clock period. An alarm circuit 66 energizes a LED L3 when the receiver clock deviates from twice transmission clock frequency by comparing information bits with the output of a frequency divider 165.  Where alternate frames are inactive, the receiver may incorporate a secondary transmitter for sending back a start, stop, and three data pulses, which may be commands expressed by the states of contacts (341-343, Fig. 9, not. shown) under the control of half rate pulses from the receiver clock and a cycling circuit (327). The receiver input is muted by grounding the output of LED isolator 41.  The secondary transmission is received through a LED isolating circuit (351, Fig. 10, not shown) by a secondary receiver at the primary transmitter position, which primary transmission is inhibited during send back periods by setting flip-flop (86) (Fig. 3, not shown) thus latching line relay (25) contacts closed. The secondary receiver also includes a clock oscillator (363) clocking information pulses into a register (366) which are passed into store (370) which may control indicating circuits and be provided with a master alarm. Counter (83, Fig. 3) continues to cycle and at commencement of next frame, flip-flop (86) is reset which inhibits secondary receiver Schmitt trigger circuit (354) and a further active frame is transmitted.