In its logical structure, a PROFINET field device is always modular in design. Modularity in the logical sense, however, does not require actual modularity in the electrical and mechanical design sense. An IO device is usually comprised of a communication module with Ethernet interface and (physical or virtual) modules assigned to it. The assigned modules handle the actual process data traffic. The access point for communication (Ethernet interface with data processing) is called the DAP (Device Access Point). The following structures are standardized for an IO device:

• The device model consists of slots, subslots, modules and submodules.
• The slot designates the insertion slot of a module in an IO field device. A field device usually has two or more slots.
• A module is comprised of one or more submodules or provides available subslots into which submodules can be inserted.
• The modules themselves have no task other than to provide structuring. The actual inputs and outputs (channels) are implemented in its submodules. The granularity of the channels (bitwise, bytewise or wordwise division of IO data) is determined by the manufacturer. Acyclic services always address submodules. Therefore, a module always contains at least one submodule.
• The data content of a submodule is always accompanied by status information. The index specifies the data within a submodule inserted into a slot/subslot which can be read or written acyclically using read/write services. For example, parameters can be written to a module, or manufacturer-specific module data can be read out based on an index. Specific indexes are defined in the standard here. Additional indexes can be freely defined by the manufacturer.

The submodule is the owner of the user data, diagnostics, channels, actual configuration, records and I&M data. Cyclic IO data of the submodule in the device is addressed by specifying the slot/subslot combination of the insertion slot. They can be freely defined by the manufacturer. For acyclic data communication via read/write services, an application can specify the data of the submodule to be addressed using slot, subslot and index (Figure 2).

Figure 2 – Addressing of IO data in PROFINET based on slots and subslots