Siemens SIMATIC S5 Manual page 146

Data Transmission by Accessing Layer 2 Services
SAPs and Job Numbers
The communications processor has buffers for the data to be sent and the data received. These
buffers are referred to as SAPs (Service Access Points to layer 2)
An SAP is addressed by assigning it a number. You have the SAPs with the numbers 33 to 54 and
64 (default SAP) at your disposal for invoking layer 2 services. To enable a SAP to be referenced
from the STEP 5 program, a layer 2 access must be configured for it. You define the SAP number
in DB1 for this purpose ( section 8.3)
The interaction between STEP 5 programs and SAPs is explained in Figure 8-3. Figure 8-3 is
explained on page 8-7.
Sending PLC (active)
Control
processor
Request via L2-
SEND, e.g. via
DB1
A-NR 33
STEP 5
Confirmation via
L2-RECEIVE and
program
A-NR 33
Receiving PLC (active or passive)
Control
processor
DB1
STEP 5
program
Indication via
L2-RECEIVE, e.g.
via A-NR 154
Figure 8-3. Using Job Numbers (A-NR) for Accessing Layer 2 Services via SAPs
8-6
Communications
processor
SAPs
33
.
.
.
54
64
Communications
processor
SAPs
33
.
.
.
54
64
S5-95U, SINEC L2
SINEC L2
interface
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SINEC L2
interface
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EWA 4NEB 812 6112-02
LAN
bus