data returned from the slave to the master. Thisdirectional attribute of the I/O is identical toI/O that is hardwired directly to backplane I/Oin a PLC rack. It typically maps into the inputand output areas of PLC memory, as shown inFigure 1-5, and can generally be accessed by thePLC logic program in exactly the same way asbackplane I/O. This cyclic (repeated) I/O dataexchange takes place asynchronously to the controllogic scan and is repeated as quickly as possible.Data exchange takes place every cycle for everyslave in a masters configuration. At the most commonlyused baud rate of 1,500kbit/s, dataexchangecycles are normally repeated manytimes during a single control-logic scan.BASICS OF PROFIBUS OPERATION 13Figure 1-4 Master/Slave Data ExchangeFigure 1-5 Bus I/O Maps Into PLC MemoryAlthough 85 percent or more of PROFIBUSinstallations are single-master systems, multimastersystems, as illustrated in Figure 1-6, existand work quite well. In such a system, each masteris given control of the bus for a short timeand during this time it exchanges I/O data witheach of its assigned slaves. It then passes controlto the next master on the bus, via a short mes-