Protocol Data Unit (PDU)
A packet of data passed across a network via telegrams. The
term implies a specific layer of the OSI seven layer model and
a specific protocol. Each layer has its own PDU that is
extended subsequently from the physical layer up to the
application layer:
•
Physical layer protocol data unit (PhPDU);
•
Data link protocol data unit (DLPDU);
•
Application protocol data unit (APDU).
Slave Parametrization
For a DP Slave several levels of parametrization exist.
(1) The parameters on the DP communication level can be
defined via a GSD file and comprise features such as baud
rates, timing constraints, identification, options, transferable
data structures, publisher subscriber links, etc. This level
supports parametrization of simple modular Slaves and also
special common additional communication layers such as
PROFIsafe. This parametrization is fixed for a given operational
life cycle after start-up.
(2) More complex devices may be parametrized via EDD and/or
FDT/DTM technology via an acyclic communication service
(MS2).
(3) For parameter changes at run-time such as batch
operation (recipes) or motion control, special "parameter
channels" associated with the cyclic data structures may be
added or the MS1 services together with proxy function blocks
may be used.
State Machine (DP)
An abstract machine consisting of a set of states (including
the initial state), a set of input events, a set of output events,
and a state transition function. A state machine describes the
behaviour of a field device how to react in different situations.
The state machine for DP Slaves comprises the following
states/actions:
- Power_On_Reset --> Set Slave address --> if successful, a
transition follows to:
- Wait_Prm --> Parametrization, diagnosis (optional) --> if
successful, a transition follows to:
- Wait_Cfg --> Configuration, diagnosis (optional) --> if
successful, a transition follows to:
- Data_Exch --> Normal operation: cyclic data exchange.
On top of this basic communication layer state machine
application profiles are defining their own additional state
machines, e.g. PA devices, PROFIdrive, PROFIsafe, Ident
Systems, Weighing and Dosage Systems.
State machines are best modelled and documented with the
help of the "Unified Modelling Language (UML)".
Station Address
Within PROFIBUS DP the address of a communication
participant (Master or Slave). The permitted range is 0 to 127,
with:
- 126 intended to be used for the "soft" addressing of Slave
devices;
- 127 intended to be used for broadcast messages to all the
