Data Transmission By Accessing Layer 2 Services - Siemens SIMATIC S5 Manual

S5-95U, SINEC L2
8 Data Transmission by Accessing Layer 2
Services
In this chapter, you will learn
• the basic principles underlying this type of transmission,
• how to assign the PLC's parameters, and
• what the STEP 5 programs for this type of transmission look like (examples).
The S5-95U with integral SINEC L2 interface has a control processor and a communications
processor.
Certain parts of the communications section have defined designations.
The SINEC L2 transmission technology is referred to as "layer 1". The SINEC L2 interface of the
S5-95U is implemented in RS 485 technology ( section 1.6).
The operating system software of the communications processor is referred to as "layer 2". Some
of these operating system programs (layer 2 services) can be invoked direct from the STEP 55
program.
The layer 2 services are also referred to as FDL (Fieldbus Data Link) services.
A layer 2 access must be activated before a layer 2 service can be invoked. You do this by
assigning parameters in DB1 and programming a call in the control program. You can invoke several
layer 2 services or invoke the same service more than once simultaneously. In this case, you must
program several layer 2 accesses in DB1.
Data transmission by accessing layer 2 services is ideal for communications between S5-95Us and
PROFIBUS-compatible programmable controllers or field devices that do not support the standard
connection, PLC-to-PLC connection or cyclic I/O types of transmission.
S5-95U programmable controllers can also communicate with each other over layer 2 accesses, in
which case the user-friendly standard connection, PLC-to-PLC connection or cyclic I/O types of
transmission can be used ( chapters 4, 6 and 7). Compared with the types of data transmission
mentioned above, the programming overhead for layer 2 accesses is high.
EWA 4NEB 812 6112-02

Data Transmission by Accessing Layer 2 Services

8-1