A placeholder in the Library Manager is a reference on a definite library version. Placeholders simplify the maintenance of library hierarchies. In the CODESYS environment there are the following places where you can define placeholders.
Device Description: For device-dependent libraries (Depending on the runtime system version)
Library Profile: For CODESYS features (Depending on the compiler version)
Application: For so called “unbound” placeholders. CODESYS takes care of the correct handling of this “unbound” placeholders, which are neither defined in a device description nor in a library profile.
Consequent usage of placeholders allows to change libraries at a lower level of the dependencies hierarchy (e.g. update, bug-fix, …) without the need to adapt all libraries of the higher levels or to adapt the device descriptions.
For more information see also the descriptions of the various possibilities to reference a Library.
The search order is as follows:
Application related “unbound” Placeholders
Device Description
Library Profile
It is always possible to overwrite the resolution of a placeholder in the application context.
The namespace of a library can be changed in the context of an library manager. But the name of a placeholder can not be changed and therefore it is highly recommended to select a name with a high probability that no other one will select this again.