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Extensive use is made of shift registers in the RPC system. Serial in Parallel out (SiPo) registers are used to receive data from the PC and deliver it as discrete output to the layout. While Parallel in Serial out (PiSo) registers receive discrete inputs from sensors and the like and deliver them to the PC as a data stream. Below is Fig.7 extracted from TB G16_22 which has been split in two parts.