Siemens SIMATIC S5 Manual page 147

S5-95U, SINEC L2
Explanation of Figure 8-3:
A request is passed to SAP 33 from the STEP 5 program by L2-SEND.
Job number (A-NR) 33 indicates that L2-SEND accesses SAP 33.
In the case of a request, the following always applies:
The communications processor sends the net data of the request to the receiver and deposits them
in the SAP. This SAP number (destination SAP) must be specified in the request header ("36
SAP number 54 in the example). If you select the default SAP (SAP 64) as the destination SAP, you
must specify "F " in the request header for SAP number 64.
H
The receiver can accept the indication (data received) from SAP 54 and write it into the STEP 5
program. L2-RECEIVE fetches the indication with job number 154.
In the case of an indication, the following always applies:
The sender receives a confirmation via the layer 2 service processed. L2-RECEIVE fetches the
confirmation with job number 33.
In the case of a confirmation, the following always applies:
You define the job numbers when assigning the parameters to L2-SEND and L2-RECEIVE (
chapter 5)
Storing the Send and Receive Data
The frame to be sent (request) consists of the 8-byte header (bytes 0 to 7) and the net data of up
to 242 bytes.
The following information must be entered in the request header by the STEP 5 program:
• The type of layer 2 service requested
(SDA, SDN, SRD, RUP_SINGLE or RUP_MULTIPLE)
• The priority of the send message
("low" or "high";
• The SAP number of the receiver (= destination SAP)
• The address of the destination station
The communications processor confirms the request automatically. The confirmation header
contains new information in the form of the link_status, which indicates whether the job has been
carried out correctly or with error(s).
In the case of the layer 2 service SRD, the confirmation consists of the header and up to 242 bytes
of net data.
The indication (frame received) consists of the 8-byte header (bytes 0 to 7) with the address of the
source station and up to 242 bytes of net data.
Figure 8-4 shows the general structure of a request (frame to be sent), confirmation and indication
(frame received), in each case with header. The designation of the header bytes conforms to the
PROFIBUS standard.
EWA 4NEB 812 6112-02
Section 1.3 "Time Management of the Network")
Data Transmission by Accessing Layer 2 Services
Job number = SAP number
Job number = SAP number + 100
Job number = SAP number
" for
H
8-7