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IBM System/7 Functional Characteristics - All about the IBM 1130 ...

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Command SequencesThe 2790 Control must transmit several active frame cycles to perform a single operation.Figures 14-7 and 14-8 show read and write command sequences, respectively, and <strong>the</strong>proper area station responses to <strong>the</strong> commands. Critical timing conditions may exist forcertain 2790 devices as shown in Figure 14-6.For example, when no area stations require servicing, a frame containing an any-addresscode in <strong>the</strong> area station address byte and a read null command in <strong>the</strong> control byte istransmitted on <strong>the</strong> loop (see Figure 14-7).BeginSC* 02Read nullRequestDataTransferDataTransferEnd DataTransfer IIdleAS** 12AS 62AS 72AS 42Read reqRead data reqRead end reqRead null ack(First datatransfer)SC 06 SC OA SC OEReadRead dataRead endAS 46AS 6AAS 3ERead corn ackRead data ackRead end ack(AS entersdata mode)(Second datatransfer, samedata as above)>•*SC = 2790 control program command**AS = area station response*** = hexadecimal codeFigure 14-7. 2790 read sequences14-26 GA34-0003

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