Patent GB-1525623-A (GB)
Title: Data transmission system
IPC: E21B47/12
Filed: 1975-08-12
Abstract: 1525623 Multiplex communication GENERAL ELECTRIC CO 12 Aug 1975 [21 Aug 1974] 33519/75 Heading H4M [Also in Division G3] In apparatus for communicating information from a plurality of data sources (instruments 212) located in a borehole to a control unit at a different location, via a cable in the borehole, a multiplexer 219 receives the signals from the sources and connects them in sequence to an output 256 under the control of a sequence controller 224. The multiplexer output is connected to an A/D converter 236 and the digital signals from the A/D converter have a direct current offset level subtracted from them in subtractor 240. The subtractor signals pass to the sequence controller 224, wherein address signals identifying the multiplexer inputs and thus the data sources are associated with the multiplexer output signals. Thereafter the sequence of address and data signals are applied (via a line driver) to the cable in the borehole. At the different location, the control unit responds to the address and data signals to provide indications of the information from and identity of the data sources. At the surface location the signals from the instruments 212 may be supplied to an on line computer 118 (Fig. 2) to determine drilling commands. The cable may be the power cable in the borehole and the line driver may operate by altering the power factor of the supply on the cable in accordance with the "1" or "0" digit to be transmitted. In addition to generating address signals identifying the data sources, reference voltage source and ground connection forming the various multiplexer inputs, the sequence control 224 may provide address signals identifying the various motors controlling the drilling operations in the borehole. These address signals are associated with zero data and passed in sequence to the line driver and also applied to a motor control circuit (72, Fig. 1, not shown). When a motor control switch-on command is received from the cable by a line receiver (82) it halts operation of the sequence control and activates the selected motor via the motor control circuit. At the surface location, signals from data sources, identified by their addresses, are applied via receiver 86 to shift register 96 under the control of sync detector 92 and thereafter to computer 118 and readout arrangements 130. The addresses identifying motor channels are decoded at 110. Assuming that the appropriate motor has been selected to operate by closing the switch 151-154, a signal is applied to modulator 156 to control the power switch 158, such that e.g. a negative power half cycle is omitted. This omission is detected in the borehole by the line receiver (82) and the motor starts operating. It is stopped by a signal from the computer circuits after the evaluation of the monitored data which is applied via amplifier 161 and causes e.g. a positive power half cycle to be omitted. To compensate for zero offsets in the individual source output signals, each line is first switched to its natural zero level, and the resulting digitized value stored in to data-latch 238. Subsequently this value is subtracted from the measured value passing to the subtractor via line 250.