Patent GB-1004170-A (GB)
Title: Control system with time reference for data acquisition
IPC: G05B19/18
Filed: 1964-07-09
Abstract: 1,004,170. Selective signalling. MOTOROLA Inc. July 9, 1964 [Aug. 5, 1963], No. 28403/64. Heading G4H. In a system for centrally monitoring a plurality of remote stations, each of which may measure locally a number of different parameters, such as pressure, temperature &c., the central station can first transmit a " freeze " command which causes every remote station to store the values of its parameters at the instant the freeze command is received, and it can then sequentially call upon the remote stations to transmit the stored values of their parameters, whereby the central station eventually receives the values of all parameters as they were when the freeze command was transmitted. A detailed description of the entire system is not given but a few isolated features are described. Data.-Three frequencies are used for transmitting binary bits, the centre frequencywhich occurs between each bit, being referred to as a " neuter." Bits are notionally grouped into fives and are given significance either by a 2-out-of-5 code or the pure-binary code, or each bit is given an independent significance, for example when signalling the states of five two-position switches. Variable length words are used generally, but " housekeeping " instructions, of which the freeze command, Fig. 4, is an example, comprise fifteen bits-construed on a 2-out-of-5 code-and each is repeated for checking purposes. The 'freeze " command.-The freeze command causes all remote stations that receive it correctly to digitize and store the present values of their parameters. In Fig. 2, remote station C has not received the freeze command connectly (because of fade or noise for example) and therefore takes no action. The stations are then interrogated sequentially to transmit their parameter values but station C gives no response. At each remote station when a freeze command is received, a local " clock " is started which clears all the stores after a certain time irrespectively as to whether they have been interrogated or not. This ensures that old data is not transmitted to the central station. The freeze command when received at a remote station is stepped into three five-bit shift registers 121, 122, 123, Fig. 9, as described in Specification 981,411, which is referred to, and as the second command enters the registers it is compared bit-by-bit with the first command which it displaces. Checks are also made that thirty bits are received and that there are only two onebits in each group of five. If the checks are satisfied the command is decoded to mark one of four leads 90, 91, 92, 93 in accordance with its value. Code 91 only is a freeze command and the others are other housekeeping commands. The " smear " command.-The system described also has the feature that when certain changes occur at a remote station, for example when switches close or open, they are immediately signalled to the central station. This may however, cause garbling at the central station if such transmissions overlap, and is overcome by the use of a " smear " command. The smear command is effective at all remote stations to cause them to transmit, after delays corresponding individually to the remote stations, the states of the equipment in which changes might occur. In this way the stations transmit individually in succession.