Siemens SIMATIC S5 Manual page 159

S5-95U, SINEC L2
8.6 Sending Data to Several Stations (SDN)
The SDA (Send Data with No Acknowledge) layer 2 service is used if an active station is to send
data to several active or passive stations (multicasting).
Data can also be sent to one active or passive station with the SDN service. However, SDA is
better suited for this ( section 8.5).
SDN has the following disadvantage compared with SDA:
The confirmation received by the sender contains no information on whether the data have been
correctly received or not. The sender only receives confirmation from its local communications
processor that the data have been sent off properly.
Prerequisites for multicasting:
• You must assign the same receive SAP in the DB1s of all receiving stations ( section 8.3).
• You must enter the global address for multicasting "127" in byte 6 of the request header.
Data transmission schematic ( Figure 8-8):
The request (header + net data to be transmitted) in the flag or data area is sent with L2-SEND
(request to layer 2).
The 'Receive" status byte informs all receivers that an indication has arrived.
Each receiver fetches the indication (header + net data received) with L2-RECEIVE.
The 'Send' status byte informs the sender that a confirmation has been transmitted.
The sender fetches the confirmation (= header) with L2-RECEIVE.
S5-95U
Sender control program (Station 1 in the
Request SDN SEND
Header with 8 bytes
In example: DB33,
DW1 to DW4
Data
In example: DB33,
DW 5 to DW 8
STBS:...3
Confirmation header
8 bytes
In example: DB33,
DW 10 to DW 13
Figure 8-8. Sending and Receiving Data without Acknowledgement or Confirmation (SDN)
EWA 4NEB 812 6112-02
Request
SDN request
(Data)
STBS:...2
H
Confirmation
H
(acknowl-
edge)
RECEIVE
STBS:...4
H
Data Transmission by Accessing Layer 2 Services
LAN Bus
Receiver control program (Station 2 in the
Indication
(Data)
S5-95U
STBR:...1
H
Indication header
RECEIVE
8 bytes
In example: DB34,
STBR:...4
H
DW 10 to DW 13
Data
In example: DB34,
DW 14 to DW 17
8-19