GSDML Modules, Submodules, Slots, Subslots
A PROFINET device does not present its data as a flat block - it is organized as modules plugged into slots and submodules into subslots, and the GSDML file is the map. Understanding that structure is what lets you configure a device correctly and know exactly where a value lives. This page explains the module and slot addressing model that GSDML describes.
GSDML Module and Submodule in one line: A GSDML file describes a PROFINET device as a set of modules that plug into numbered slots, each module holding submodules that plug into subslots. A slot is a position in the device; a module is what occupies it; a submodule is a functional unit inside the module with its own I/O. The controller addresses every input and output by its slot and subslot, which is why the module layout must match the configuration exactly.
Slots and Modules, Subslots and Submodules
The model borrows the language of a physical rack even for devices that are one solid box. A slot is a position, numbered, where a module can go. A module is the thing that occupies a slot - an input card, an output card, a functional block. Inside a module are subslots, and a submodule occupies a subslot: submodules are the finest functional units, each carrying its own inputs, outputs, or both. So the address of any I/O point is, in effect, which slot, which subslot, and which channel within it.
For a modular device like a remote I/O head with pluggable cards, this maps directly to hardware: slot 1 is the first card position, and the module in it is whatever card you fitted. For a compact device with fixed I/O, the same slot and subslot structure still exists logically, defined in the GSDML, even though nothing is physically pluggable. Either way, the engineering tool reads the GSDML to know what modules are allowed in which slots and what data each submodule provides, which is why the right GSDML device description must be imported before configuration.
Why the Layout Must Match Exactly
The controller's configuration is a plan of which module sits in which slot and which submodule in which subslot, and at connection setup it checks the device against that plan. If the actual layout differs - the wrong card in a slot, a module in an unexpected slot, a missing submodule - the application relation fails to establish or the device reports a configuration mismatch, because the controller cannot safely address data whose position it does not agree on. This is the same class of check that catches a device presenting a different arrangement than the project expects.
The addressing is the reason the match must be exact: the controller reads and writes each value by its slot and subslot, so a value in the wrong subslot is simply the wrong value. This is closely analogous to how a structured network element must be configured consistently at both ends. Once the layout matches, each submodule's I/O maps to controller addresses and then to named tags, and those tags are what a monitoring platform such as Merobix trends. Merobix records the controller's named tags; it does not itself parse GSDML or address subslots, but the accuracy of every tag traces back to a correct module layout.
Frequently Asked Questions
What is the difference between a module and a submodule in PROFINET?
A module occupies a slot - a position in the device, like a card slot - while a submodule occupies a subslot inside a module and is the finest functional unit, carrying its own inputs or outputs. The controller addresses each I/O point by its slot and subslot, so the module holds submodules, and the submodules hold the actual data.
Do compact devices have slots and subslots?
Yes, logically. Even a fixed-I/O device with nothing physically pluggable still has a slot and subslot structure defined in its GSDML file, because that is how the controller addresses its data. The rack-like language of slots and modules applies to modular head stations with real card positions and to compact devices with a fixed internal layout alike.
Why does the module layout have to match the configuration?
Because the controller addresses every value by its slot and subslot. If the actual module or submodule layout differs from the configured plan, the controller cannot safely address the data, so the application relation fails or the device reports a configuration mismatch. A value in the wrong subslot is simply the wrong value, which is why the layouts must agree exactly.
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