Siemens SIMATIC S5 Manual page 143

S5-95U, SINEC L2
Sending PLC
Active station on the LAN bus
'Send' status byte for job
number
(in flag area)
Send allowed?
Yes
L2-SEND
Job number
Mailbox
DB 41
DB 22
DB 33
Request header
Net data
0 to 242 bytes
LAN bus
Explanation of Figure 8-2:
If there are no longer any jobs being executed over a layer 2 access, the 'Send' status byte
indicates that sending is permissible. L2-SEND is invoked in the STEP 5 program (request).
L2-SEND receives the following information in the parameters passed to it:
• Job number of the layer 2 access used (programmed in DB1)
• The mailbox in which an 8-bit header and the net data to be transmitted are located.
The net data are send to the addressed receiver over the LAN. At the receiver end, a bit in the
'Receive' status byte indicates that data have arrived. The data (in the form of an indication) are
fetched from the receiver with L2-RECEIVE.
Each request is confirmed. The confirmation header indicates whether the job has been executed
with or without error(s).
(In the case of the SRD service, the confirmation consists of the header and net data requested by
the receiver.)
The "receive viable" bit is set in the 'Send' status byte of the transmitter. The confirmation is
fetched with L2-RECEIVE.
EWA 4NEB 812 6112-02
e.g. S5-95U
'Send' status byte for job
number
(in flag area)
No
Receive viable?
Yes
L2-RECEIVE
Job number
Mailbox
DB 42
DB 23
DB 34
Confirmation header
Net data 0 to 242
bytes in the case of
SRD service
Figure 8-2. Function Model Using Layer 2 Services
Data Transmission by Accessing Layer 2 Services
Receiving PLC
Active or passive station on the LAN bus
No
e.g. S5-95U
'Receive' status byte for
job number
(in flag area)
No
Receive viable?
Yes
L2-RECEIVE
Job number
Mailbox
DB 45
DB 24
DB 34
Indication header
Net data
0 to 242 bytes
8-3