Technical data

8.1.2 How Data Transmission by Direct Access to Layer 2 Services
Functions
The first 8 bytes of the block of data to be transmitted (in the header)
contain control information for the layer 2 firmware. The receiver can also
evaluate the first 8 bytes of the received block of data as status information
(e.g. error messages (link_status)).
With the data transmission services SDA, SDN and SRD, the CP 5430
TF/CP 5431 FMS uses the control information from the header of the data
to "package" the transmitted data in a frame which is then transmitted by
the SINEC L2 bus.
The basic sequence of communication via the free layer 2 access is
illustrated in Fig. 8.2.
Value of
link_status
Abbreviation
PROFIBUS
Meaning
SRD (Indication)
20
H
LO in this SRD exchange, the reply was with low priority
data.
21
H
HI in this SRD exchange, the reply was with high priority
data.
22
H
NO_DATA in this SRD exchange, no reply data were sent
Table 8.2 Meaning of the Values in Byte 3 (Link Status) in the Indication Header
B8976060/02 Free Layer 2 Communication
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