Patent GB-1314264-A (GB)
Title: Data transmission system
IPC: H04Q9/12
Filed: 1970-04-28
Abstract: 1314264 Data transmission systems SCHLUMBERGER INSTRUMENTS ET SYSTEMES 28 April 1970 [29 April 1969] 20310/70 Heading G4H A data transmission system comprises a central station and a number of remote data points each capable of being put into communication with the central station in response to the transmission from the central station of combinations of frequencies, the data points being grouped in stations each having n filters capable of detecting n of the N frequencies, the selection of a data point involving the steps of first selecting the appropriate station by transmitting at least (p + 1) of the n frequencies then selecting the desired point by transmitting p of the n frequencies. The central station and all the remote stations are connected in parallel to four lines, two of which are call lines and two response lines. The station shown in Fig. 2 has a sensor 25 with a nine-bit digital output, a sensor 26 with an analogue output, a group 27 of three indicators and a control element 30. Sensors 25 and 26 and indicators 27 give data on the operation of an industrial process to be controlled, the control being effected through element 30. The call line is connected to an input transformer 31 and a variable gain amplifier 32. The incoming signal passes to seven bandpass filters 33 to 39 tuned to frequencies F1, F12, F3, F5, F7, F9 and F11. The outputs are connected through detectors 41-47 and amplifiers 50-56. The address of the station shown is represented by the four frequencies F7, F9, F11, F12. When these are received a pulse appears at the output of gate 57 which sets flip-flop 61, thereby closing relay 62; closes the address relay 91, which sends back a coded number indicating the number of datapoints; transfers data from sensor 25 into store 90; and triggers flip-flop 60 to enable all the transmission gates 63-67 so that the data points may be selected to send their information. If the three frequencies F7, F9 and F11 are received gate 63 gives an output closing response relay 70. If all the indicators 27 are closed contacts 80 allow the three frequencies F7, F9, F11 to be passed through relay 70 to the response line. If any are not closed the corresponding frequency is not sent back. If the three frequencies F5, F9 and F11 are received gate 64 closes relay 71 and the analogue signal of sensor 26 converted at 82 to a frequency is sent back on the response line. If frequencies F3, F9 and F11 are received the first three bits of the data from sensor 25 will be sent back to the central station in the form of a combination of some or all of these frequencies. Other combinations of frequencies each cause one of the other six bits to be sent back. When all the data -points have been scanned in this way frequency F1 is sent from the central station to reset flip-flops 60 and 61. Control element 30 is operated by sending first the address as before, then the combination F7, F9, F11 for a long duration. This is detected by discriminator 93 which triggers relay 94 and operates the element 30. Of the seven filters, F1 and F12 are used only for resetting and addressing the station. From the remaining five, ten combinations of three can be formed each of which may select a different data point. Alternatively 30 remote indicators 27 and ten controls 30 may be selected. Ten addresses can be formed using the same ten combinations and F12. From ten frequencies available at the central station 6 different groups of five can be selected. The maximum capacity of the station is therefore 60 stations each having 10 data points. The central station has a scanner capable of operating in three modes: a manual mode, whereby the address of a particular data point is selected; a semi-automatic mode, whereby all the data points of a given station are scanned in turn, and an automatic mode which scans all the data points of all the stations.