The FDL protocol is the basic protocol on which every other PROFIBUS protocol is based. A great advantage of the protocol is its speed. Its proximity to hardware provides a correspondingly high message throughput rate. Another advantage is its flexibility. With its 4 services (of which SIMATIC S7 offers two as interfaces) it covers a broad spectrum of transfer options. The FDL protocol is supported by SIMATIC S7, S5 and PC systems each in connection with a PROFIBUS CP. Furthermore, SIMATIC S5 95U with PB interface also supports FDL communication.
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.
Types of data transfer
FDL communication supports the following transfer services:
- SDA - Send Data with Acknowledge
- SDN - Send Data with no Acknowledge
- SRD - Send and Request Data with Acknowledge
- CSRD - Cyclic Send and Request Data
Only the SDA and SDN services are supported in the SIMATIC environment.
FDL connection types (in the SIMATIC configuration NetPro)
|Specified FDL connection
||Local nodes and connection partners are defined by connection configuration. Receipt of data is acknowledged.|
|Unspecified FDL connection
(point-to-point with free Layer 2 access)
||The local node is defined by connection configuration. The connection partner is addressed in the user program. Receipt of data is acknowledged.|
||An active node sends a message to all other nodes. Receipt of data is not acknowledged.|
||An active node sends a message to a group of nodes. Receipt of data is not acknowledged.|
Performance features of FDL communication
*1) The connection partner is entered in the job header. The header consists of 4 bytes.
|Max. data length of a specified FDL connection (SDA)
|Max. data length of an unspecified FDL connection (SDA)
|Max. data length of a Broadcast or Multicast connection (SDN)
|Possible address areas
||E, A, M, D
||E, A, M, D|
|Max. number of connections
||Depends on CP*2)
||Depends on CP*2)|
|Block types of services
*2) The max. number of connections that can be operated depends on the CP used, the CPU used and the number of connections in the multiprotocol mode.
|Data area per job
||1 - 240 bytes
||1 - 236 bytes|
|Number of possible connections
||Up to 16 per S7-300 CP
Up to 64 per S7-400 CP
|Up to 16 per S7-300 CP|
Up to 64 per S7-400 CP
Advantages of the FDL connection
- Efficient and fast data transfer through slim architecture.
- Easy connection of S5 and third-party systems to SIMATIC S7.
Disadvantages of the FDL connection
- Additional resources in the user program for monitoring the data transfer and error messages of the communication blocks, as well as data management (the CP is the connection endpoint, i.e. the FDL protocol acknowledges the data only up to the CP layer. The user program is responsible for saving the data received).
- Only suitable for transferring small data volumes (<=240 bytes)
- General information on communication via SIMATIC S7 is available in Entry ID: 20982954.
- Entry ID: 20987711 includes a complete example of FDL communication with SDA service via PROFIBUS CP and S/R interface.