Patent GB-1503381-A (GB)
Title: Data system
IPC: G09G1/00
Filed: 1975-03-03
Abstract: 1503381 Data processing ONTEL CORP 3 March 1975 [6 March 1974] 8734/75 Heading G4A A data system comprises a plurality of data devices, e.g. 10, 11, a plurality of respective device controllers e.g. 16, 17, a data memory 32, and a CPU 26, first memory portions within data memory 32 being assigned to the respective device controllers for storing address information, and second memory portions within data memory 32 being assigned to the respective device controllers for storing control information, a multiplexer 22 interrogating the first memory portion assigned to a selected device controller to obtain and store the address of a location within data memory 32 to or from which data is to be transferred, a data character being transferred between a data buffer coupled to the selected device controller and the location within the data memory 32 identified by the address stored in multiplexer 22, the multiplexer 22 interrogating the second memory portion assigned to the selected device controller to obtain and store control information pertaining to the data transfer, this control information being used, inter alia, to detect the termination of a transferred data message. Termination of a data message may be detected by comparing each transferred character with a stored control character which is used to indicate the end of a message, or by comparing the address of the location used to store each transferred character with a stored control address indicating the maximum address in data memory 32 reserved for the message, or by counting the number of transferred characters and comparing this with a stored count. The invention is embodied in the figure in the form of a plurality of business machines 15. Each illustrated data device 11 is a local device with regard to the illustrated business machine 15, whereas the illustrated data device 10 is a remote device connected by a communications line 12 and a modem 18; the devices 10, 11 are I/O devices, e.g. teletypewriters or printers. In the event that a remote data device 10 wishes to communicate with CPU 26, an Attention character is sent on line 12 to all business machines 15, the address of the desired business machine 15 then being sent to select that business machine. This is followed by a Start of Text character, the data characters, an End of Text character, and a Block Check character used to check the transmitted data message. Each transmitted character may be accompanied by a parity check bit. Four successive locations of data memory 32 may store respectively the address of the first byte of the message, the address of the most recently transferred byte (incremented by 1 with each character transferred), the address of the final byte to be transferred, and the control character indicating termination of a message. Simultaneous access requests from two or more device controllers to the CPU 26 are resolved by means of a priority selection circuit (106), Fig. 5 (not shown), which sends an Acknowledge signal to the selected device controller. A memory access sequencer (110), Fig. 6 (not shown), then interrogates the appropriate first and second portions of the data memory 32 in the correct sequence. A detailed circuit diagram of a remote device controller 16 is given in Fig. 7 (not shown). The circuit of a local device controller 17 is similar except that the part required for handling the Attention character is omitted. Data received from a data device 10, 11 or processed by the CPU 26 may be displayed on a CRT 50.