that a message is designated for it andthen respond to the sender. Hence, each deviceon a PROFIBUS network must have an assignedaddress. For specifying the address, most deviceshave either rotary switches (decimal or hexadecimal)or DIP switches. Some few devices requirethat their address be set across the bus using aconfiguration tool. This concept will be discussedin the next chapter.The PROFIBUS protocol supports addressesfrom 0 to 127. However, addresses 126 and 127have special uses (discussed in Chapter 2) and6 BASICS OF PROFIBUS OPERATIONmay not be assigned to operational devices.Address 0 has become something of a defaultaddress that vendors assign to network configurationand/or programming tools attached to thebus. Thus, the addresses that may be used inpractice for operational devices for example,PLCs, I/O nodes, drives, encoders, and the like are 1 to 125.1.2 Device and System StartupThe user specifies which slave devices the mastershould find on the bus as well as what informationis to be transferred from the master to eachslave during this startup phase. (Relaxthis ismuch easier than you might think!) All of theinformation that the master must know to startup the bus comes from a configuration databasefile that is generated by a PROFIBUS configurationtool.Each vendor of PROFIBUS master devices offersa configuration tool for generating the databasefile for their masters. However, once one haslearned how to use any of these tools, it is generallyquite easy to transfer this knowledge toanother tool because all PROFIBUS configura-BASICS OF PROFIBUS OPERATION 7tion tools must share certain common functionality.A configuration tool for cyclic I/Ooperation must be able to do the following: process GSD (device description) files andmaintain a hardware catalog of devices tobe configured on the bus allow the PROFIBUS device address to bespecified allow the specification of the input and outputdata to be transferred between masterand slave allow certain startup parameters to beselected in order to activate specific operatingmodes or features of the device allow selection of the system baud rate generate the database file so it can be usedby the masterAt the same time a vendor develops a slavedevice, it must develop a device description(GSD) file. This file completely describes thePROFIBUS functionality of the device forexample, baud rates supported, possible input/8 BASICS OF PROFIBUS OPERATIONoutput data configurations, startup parameterchoices, and so on. These GSD files can typicallybe downloaded via the Internet either fromwww.profibus.com or from an individual vendorsweb site. Once you install the GSD filefor a device into the PROFIBUS configurationtool, it will appear in the tools hardware catalog,which enables it to be configured for bus operation.The installation process varies for different configurationtools, although it is extremely simplein any case. In some tools, installation consistsof nothing more than copying the GSD file intoa gsd subdirectory, while in others one simplyimports the new GSD file into the hardwarecatalog by selecting that option from a menu.Once all the appropriate GSD files are installedinto the configuration tool, you can define a busconfiguration. This is a straightforward process.You first pick the appropriate master from themaster device list in the hardware catalog andassign a PROFIBUS address. Not surprisingly,you will generally see only master devices from aspecific vendor within that vendors configurationtool! This is nothing to be concerned