Patent GB-1516669-A (GB)
Title: Switches and to data communications systems
IPC: G06F13/38
Filed: 1975-11-10
Abstract: 1516669 Distributers WESTERN ELECTRIC CO Inc 10 Nov 1975 [14 Nov 1974] 46438/75 Heading H4M [Also in Division G4] A switch includes means responsive to any one of a plurality of control signals at an input of said switch for causing a signal path to be established between said input and a respective one of a plurality of outputs of said switch, and means responsive to said control signal for inhibiting said causing means from being enabled by signals on said signal path, said inhibiting means being adapted to be disabled in response to the absence of signals on said signal path for a predetermined interval. A central station (including a minicomputer) is connected over respective lines with data base units (memories) and multipoint switches, each multipoint switch being connected over respective lines with remote terminals (or with further multipoint switches connected over respective lines with remote terminals), bits being sent in FSK form. The central station polls the remote stations (on all its lines to multipoint switches simultaneously), a polling sequence (i.e. for one remote terminal) consisting of stop bits (ones i.e. a continuous tone), remote terminal address; stop bits (ones) and a silent interval of tl seconds. (To poll a remote terminal linked to the central station via two multipoint switches in series, two polling sequences are needed, the first containing the address of the multipoint switch nearer the remote terminal, and the second containing the address of the remote terminal). The multipoint switch sets the address bits into flip-flops, and decodes them to select a line to remote terminal (or further multipoint switch) which line is thus connected to the line from the central station and receives the stop bits following the address. The multipoint switch also blinds itself to signals on the path thus connected between the central station and remote terminal (or further multipoint switch) until there has been no signal on this path for t2 seconds. A remote terminal will receive the stop bits which followed its address. followed by silence, and recognizes this combination as a valid poll. If it requires data from one of the data bases, the remote terminal will then send over the path to the central station stop bits followed by control characters and text characters. The central station will start to receive this enquiry message before expiry of the t I interval, and after this message has been received, will leave a silent interval of t1 seconds before polling the next remote terminal. When the central station has obtained the required information from the data base, then the next time the remote terminal in question is polled, the central station sends a reply message containing the information to the remote terminal, the message consisting of stop bits, remote terminal address, stop bits, control characters and text characters (assuming only one multipoint switch is involved). Alternatively, if the data base is unavailable, the reply message tells the remote terminal operator, so. If the central station detects an error in the enquiry message, it does not wait for the next time the remote terminal is polled, but as soon as it can sends a "retry" reply to the remote terminal operator (this reply lacks the address, the path to the remote terminal still existing). Every reply message includes a control character which prevents erroneous alteration of the address stored in the multipoint switch (in case the latter is not blind). On receiving a reply message, a remote terminal will tell its operator to retry, on detection of any of the following errors: wrong address, character parity error, longitudinal parity error (using an LRC character in the message), loss of carrier. In polling one remote terminal after another, the just-polled terminal will receive the address of the next preceded by stop bits since the multipoint switch changes over in response to the address, but this is not taken as a valid poll by the former terminal since the stop bits are not followed by silence. Addresses each include a bit to prevent the wrong multipoint switch responding in the cases where there are two in series between the central station and remote terminal. Each multipoint switch includes straps to enable it to be set up as the first or second in a central-remote link. Each multipoint switch places itself in an idle state (opening all lines to remote terminals and to further multipoint switches nearer remote terminals) on detecting parity error in an address and on detecting continuous transmission (in either direction) for more than t3 seconds, and has clock-driven counters for measuring out the intervals t2 and t3. Each remote terminal generates character parity bits and an LRC character for each message it transmits. The system can be used for credit checking and alarm polling.