At field level, protocols are not suitable for the PROFIBUS with a large number of services or complicated data processing, because the bus cycle time and response time required cannot be reached.
The PROFIBUS-DP protocol (distributed I/O) was developed specially to cover the field level in the automation hierarchy. The basic feature of the PROFIBUS-DP is that user data is displayed in the form of a cyclic data image. Here, object-oriented interfaces as used in the FMS or S7 protocol are avoided completely. The principle of the PROFIBUS-DP communication is a master-slave system. A master polls one or more slaves cyclically.
Instead of the user interface at Layer 7 (ISO-OSI reference model) of the DP protocol, there is the user interface that is based directly on Layer 2 (ISO-OSI reference model), i.e. on FDL, as standard application together with the DDLM (Direct Data Link Mapper).
There are two different types of DP master:
- Master Class 1: controls the process cyclically
- Master Class 2: device parameterization and diagnostics
Since there are no differences at Layer 2 for all the PROFIBUS protocols, all the protocols can be operated in parallel in one PROFIBUS network.
Position in the ISO-OSI reference model:
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Fig. 01 - Position in the ISO-OSI reference model
In the model, the FDL connection occupies layers 1 and 2. Layer 2 is designated the FDL layer (Fieldbus Data Link) with PROFIBUS. Above layer 2 there is another specific interface that can be assigned to layer 4. All the other layers of the OSI reference model are not marked.
The message formats of the PROFIBUS Layer 2 permit high transfer reliability for FDL communication (Hamming Distance HD=4). Messages in which errors are detected are repeated automatically.
DP protocol services:
||For reading or writing data of the parameterized distributed I/O. If a station is parameterized as slave, user data also can be made available to a master in this way.|
||For diagnosing a DP slave or DP master according to the job parameterized.|
||Permits sending of control jobs to a PROFIBUS-DP node in the following form:|
- Operating mode changes
- Read jobs for inputs/outputs of other nodes
- Sending of global control commands
Advantages of the DP protocol:
- Very fast communication protocol, due to close hardware proximity.
- Can be used with third-party systems.
- Acknowledged data transfer.
Disadvantages of the DP protocol:
- Only suitable for transferring small data volumes (<=244 bytes).
- Can only transmit static data lengths.
- Relatively high configuration workload.
General information on communication via SIMATIC S7 and other information on the DP protocol is available in Entry ID: 20982954.
DP, DP protocol, Distributed I/O, Master, Slave, PROFIBUS, DP_SEND, DP_RECV, DPWR_DAT, DPRD_DAT