Confirmed services are only permitted on connection-oriented communication relationships
while unconfirmed services may also be used on connectionless relationships. Unconfirmed
services may be transferred with either a high or a low priority.
In the Profibus standard, the interaction between requester and responder, as implemented
by the appropriate service, is described by a service primitive.
The Profibus FMS services can be divided into the following groups:
• Context management services allow establishment and release of logical
connections, as well as the rejection of inadmissible services.
• Variable access services permit access (read and write) to simple variables,
records, arrays, and variable lists.
• The domain management services enable the transmission (upload or download)
of contiguous memory blocks. The application process splits the data into
smaller segments (fragments) for transmission purposes.
• The program invocation services allow the control (start, stop, etc.) of program
execution.
• The event management services are unconfirmed services, which make the
transmission of alarm messages possible. They may be used with high or low
priority, and messages may be transmitted on broadcast or multicast communi-
cation relationships.
• The VFD support messages permit device identification and status reports.
These reports may be initiated at the discretion of individual devices, and
transmitted on broadcast or multicast communication relationships.
• The (OD) management services permit object dictionaries to be read and
written. Process objects must be listed as communication objects in an object
dictionery OD. The application process on the device must make its objects
visible and available before these can be addressed and processed by the
communication services.
As can be seen, there are large amounts of Profibus FMS application services to satisfy
the various requirements of field devices. Only a few of these (5, in fact) are mandatory
for implementation in all Profibus devices. The selection of further services depends on
the specific application and is specified in the so-called profiles.
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