GB1480782A - Data processing system - Google Patents

Abstract

1480782 Data processing system SIEMENS AG 26 July 1974 [6 Aug 1973] 33081/74 Heading G4A In a data processing system in which data is transferred between a central unit and a number of peripheral devices over a block multiplex channel as described in Specification 1,480,781 where the channel unit is unable to distinguish between "channel end" and "device end" signals, the channel unit is arranged to treat the first "end" signal received from a peripheral as a "channel end" by disconnecting the appropriate peripheral from the channel and to treat a subsequent "end" signal as a "device end" reconnect request. The arrangement allows a channel unit intrinsically unable to distinguish between "channel" and "device" end signals, and thus unable to determine when a peripheral is operating off-line and when data transfers to and from other peripherals could be made over the channel, to operate in block multiplex fashion. When the first "end" signal is received from a peripheral connected to the channel it is treated as a "channel end" signal and a special "disconnect command" provided in the channel program is accessed. The channel is thus disconnected and any further "end" signals received in the presence of the disconnect command may safely be interpreted as "device end" reconnect requests. If several such "end" signals are received they are resolved on a priority basis and the selected request is serviced by obtaining, through the disconnect command, the next channel command in the program. Start requests are processed as in Specification 1,480,781 except that each channel program begins with a disconnect command so that when, in response to the start command, the appropriate peripheral emits a "device end" reconnect request it is correctly interpreted by the channel unit and the next channel command for the peripheral is accessed as above.