PROFIBUS moves blocks of data in a cyclical manner using units called Input and/or Output (I/O) Modules. These I/O Modules are defined only on the slave devices. Masters on the network exchange information with the I/O Modules of slaves assigned to them. In essence, this means that the slave devices define what information can be exchanged in a cyclical manner on the network. Note that the naming convention for I/O modules is referenced from the master's perspective:
- Data moving from the slave to master is stored in an Input Module on the slave, and
- Data moving from the master to slave is stored in an Output Module on the slave.