Data processing system

Abstract

Abstract 1480781 Data processing system SIEMENS AG 26 July 1974 [6 Aug 1973] 33080/74 Heading G4A The Specification describes data transfers between a central unit and a number of peripherals over a channel operated in block multiplex mode and the way in which requests to start new data transfer operations are handled. A channel unit controls transfers over the channel in a time shared manner under the control of channel programs. Following completion of a channel program command in respect of data transfer between a given peripheral and the central unit the peripheral emits a "channel end" signal whether or not it is still operating (e.g. a magnetic tape searching for the next location for a transfer) and, in response to the "channel end" signal, the channel unit disconnects the peripheral to allow transfers with other peripherals to proceed in the meantime. When the peripheral has finished operating it emits a "device end" signal, which may thus coincide with the "channel end" signal (above) e.g. in the case where no further transfer is to be made with the peripheral. Assuming that the two signals did not coincide and that the "device end" signal is emitted to indicate that the peripheral is ready to continue data transfer, the channel unit treats the "device end" signal as a reconnection request. The highest priority such reconnection request is granted when the channel is subsequently disconnected from any other peripheral to which it had been connected. When a start command occurs requesting that a channel program be started to control a required data transfer it is processed by first checking whether the appropriate peripheral is in operation. If it is the request is rejected and queued for future use. If the peripheral is not operating the channel unit sends a signal to the peripheral, in the intervals between data transfers where the channel is handling transfers for another peripheral. The peripheral responds by emitting "device end" which is treated as a reconnection request (above) by the channel unit so that the required data transfer is processed in accordance with the normal priority arrangement. The arrangement is contrasted with known arrangements where start commands are, if the channel is connected and in use, invariably rejected and queued. In the present arrangement a start command is processed within the normal priority arrangements so long as the specified peripheral is not operating irrespective of the condition of the channel.

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