Technical data

The link to communication end points is via so-called channels. To
communicate via the free layer 2 access, a link must be configured using
"channels" with the link editor (Section 8.3.1).
Characteristics of the free channels
The communication start and end point of a channel between two stations
on the bus is known as a service access point (SAP). An SAP is a further
address criterion in addition to the station address. As already mentioned in
the model, a channel (SAP) is addressed by a job number (ANR). An SAP
number must be specified for each channel to be able to use layer 2
services (refer to Fig. 8.3). The assignment of ANR and local SAP is made
when configuring with the COM.
☞
Make sure that any possibility of overlapping with SAPs used
for DP is excluded.
CPU memory CP 5430
L2
frame
Header
Data
field
Firm-
ware
layer2
layer1
HDB (SEND)
HDB (RECEIVE)
Fig. 8.2 Basic Sequence of Communication using Free Layer 2 Access
Free Layer 2 Communication B8976060/02
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