Data ExchangeAfter the bus system has been started up, thenormal interaction between a master and each ofits assigned slaves is to exchange I/O data, asshown in Figure 1-4. The master, a PLC with aPROFIBUS interface, for example, sends outputdata to a slave device in its configuration. Theaddressed slave immediately responds with itsinput data. It is important to grasp this concept ofoutput data sent from the master to the slave and inputdata 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-