BECKHOFF BK3xx0 - Manual (Page 14)

Notes on the Documentation
12 Fieldbus Components
PROFIBUS - Introduction
PROFIBUS DP
In PROFIBUS DP systems, a master (PLC, PC etc.) usually communicates with a large number of slaves (I/Os,
drives etc.). Only the master may here actively access the bus (send telegrams on its own initiative), while a DP slave
only sends telegrams when it is requested to do so by a master.
DP StartUp
Before the master and slave can cyclically exchange data, the parameter and configuration data is transmitted from
the master to the slaves during the DP StartUp phase. After the parameter and configuration data has been sent, the
master interrogates the slave's diagnostic data until the slave indicates that it is ready for data exchange. Depending
on the extent of the calculations that the slave must carry out after receiving the parameter and configuration data, it
can take up to a few seconds before it is ready for data exchange. For this reason the slave possesses the following
states:
Parameter data
The parameter data is sent from the master to the slave in the SetPrmLock request telegram. The SetPrmLock
response telegram does not contain any data, and therefore consists of a single byte, the short acknowledgement.
The parameter data consists of DP parameters (e.g. the setting of the DP watchdog or checking the IdentNumber
(unique to each DP device)), of DPV1-/DPV2 parameters and of application-specific parameters that only have to be
transmitted once during the StartUp. If an error is found in the parameter data, this is indicated in the diagnostic data,
and the slave either remains in or enters the WAIT-PRM state.
Configuration data
The configuration data is sent from the master to the slave in the ChkCfg request telegram. The ChkCfg response
telegram does not contain any data, and therefore consists of a single byte, the short acknowledgement. The
configuration data describes the assignment of the DP modules to the cyclic I/O data that is to be exchanged
between the master and slave via the Data_Exchange telegram in the cyclic data exchange phase. The sequence of
the DP modules added to a slave in the DP configuration tool determines the sequence of the associated I/O data in
the Data_Exchange telegram.
Diagnostic data
The diagnostic data is requested by the master using a SlaveDiag request telegram without any data. The slave
replies with the diagnostic data in a SlaveDiag response telegram. The diagnostic data consists of the standard DP
diagnostics (e.g. the state of the slave, the IdentNumber) and of application-specific diagnostic data.
Cyclic data exchange
The heart of the PROFIBUS DP protocol is cyclic data exchange, during which the master carries out an exchange of
I/O data with every slave during a PROFIBUS DP cycle. This involves the master sending the outputs to each slave
with a DataExchange request telegram, while the slave replies with the inputs in a DataExchange response telegram.
This means that all the output and/or input data is transmitted in one telegram, in which the DP configuration (the
sequence of DP modules) specifies the assignment of the output and/or input data to the slave's actual process data.
Diagnosis during cyclic data exchange
A slave can send a diagnostics signal to the master during cyclic data exchange. In this case, the slave sets a flag in
the DataExchange response telegram, whereby the master recognises that there is new diagnostic data in the slave.
It then fetches that data in the SlaveDiag telegram. This means that diagnostic data is not transmitted to the controller
with the cyclic I/O data in real-time, but is always at least one DP cycle later.