You specify which slave devices the master should find on the bus as well as which information should be transferred from the master to each slave during this startup phase. All of the information that the master must know to start up the bus comes from a configuration database file that is generated by a PROFIBUS configuration tool. Each vendor of PROFIBUS master devices offers a configuration tool for generating the database file for their masters. However, once you have learned how to apply any of these tools, it is generally quite easy to transfer this knowledge to another tool because all PROFIBUS configuration tools must share certain common functionality. A configuration tool for cyclic I/O operation must be able to do the following:
- Process GSD (device description) files and maintain a hardware catalog of devices to be configured on the bus
- Allow the PROFIBUS device address to be specified
- Allow the specification of the input and output data to be transferred between master and slave
- Allow certain startup parameters to be selected to activate specific operating modes or features of the device
- Allow selection of the system baud rate
- Generate the database file so it can be used by the master
At the same time a vendor develops a slave device, it must develop a device description (GSD) file. This file completely describes the PROFIBUS functionality of the device, such as baud rates supported, possible input/output data configurations, startup parameter choices, and so on.
You can typically download these GSD files via the Internet either from www.profibus.com or from an individual vendor’s Web site. Once a user "installs" the GSD file for a device into the PROFIBUS configuration tool, it appears in the tool's hardware catalog, so it can be configured for bus operation.
The installation process varies for different configuration tools, but it is extremely simple. Once you have installed all of the appropriate GSD files in the configuration tool, you can define a bus configuration.
First pick the appropriate master from the master device list in the hardware catalog and assign a PROFIBUS address. You repeat the following steps until the entire bus configuration has been described: select a slave device, assign the PROFIBUS address, specify the I/O to be exchanged, and select the appropriate parameters for the desired operation of the device. You then save this bus configuration and generate the configuration database. You can now load this configuration database into the master device.
After download, the master has the information necessary to start up all the devices in its configuration. This information is stored in retentive memory. The master must now determine if the devices at the assigned addresses contained within the configuration database are physically on the bus and initialize them for "operational" or "data exchange" mode. To get the devices into this mode, a PROFIBUS master undergoes a well-defined sequence of interactions with each of the slave devices in its bus configuration. For instance, if the master device experiences a power loss, when it powers back up, it uses the configuration database in retentive memory to go through the startup sequence with each device in its configuration to get all devices back into operational mode. If a slave device fails and must be replaced, the master recognizes that a replacement device of the same type and with the same PROFIBUS address has been attached to the bus. When it does, it goes through this same startup sequence and automatically brings the device into operational mode.