Siemens SIMATIC S5 Manual page 144

Data Transmission by Accessing Layer 2 Services
Request, Indication and Confirmation
Request:
A layer 2 service is invoked from the STEP 5 program with a request. The request consists of the
request header and the net data to be sent.
The request header is interpreted by the communications processor of the sending PLC. The
request header defines the layer 2 service to be used and the destination to which the net data are
to be sent.
Indication:
The receiver receives the net data in the form of an indication. The indication consists of the
indication header and the net data received.
The indication header contains the following information on the data received:
• The layer 2 service used,
• The priority with which the data were sent, and
• The station from which the data were sent.
Confirmation:
The sender receives information on whether the layer 2 service has been executed correctly or with
error(s) in the form of a confirmation.
In the case of the layer 2 SRD service ( section 8.9), the confirmation consists of the confirmation
header and the net data received.
For all other layer 2 services ( sections 8.5 to 8.8), the confirmation consists only of an 8-byte
confirmation header.
The confirmation header tells you whether the job has been executed correctly or with error(s), and,
in the latter case, which error(s) has (have) occurred.
Section 8.2 contains a description of the types and characteristic features of the layer 2 services
and the format of the various headers.
8-4
S5-95U, SINEC L2
EWA 4NEB 812 6112-02