Siemens SIMATIC S5 Manual page 155

S5-95U, SINEC L2
8.5 Sending Data to a Station (SDA Service)
The SDA (Send Data with Acknowledge) layer 2 service is used if an active station is to send data
to an active or passive station.
Data transmission schematic ( Figure 8-6):
The request (header + net data to be transmitted) in the flag or data area is sent with L2-SEND
(= request an layer 2).
The 'Receive" status byte informs the receiver that an indication has arrived.
The 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.
Sender control program (Station 1 in the
example)
Request SDA SEND
Header with 8 bytes
SDA request
In example: DB33,
STBS:...2
DW 1 to DW 4
Data
In example: DB33,
DW 5 to DW 8
STBS:...3
Confirmation header
8 bytes
In example: DB33,
DW 10 to DW 13
Figure 8-6. Sending and Receiving Data with Acknowledgement or Confirmation (SDA)
Figure 8-7 describes the structure of requests, confirmations and indications for the SDA service.
EWA 4NEB 812 6112-02
S5-95U
Request
(Data)
H
Confirmation
H
(acknowledge)
RECEIVE
STBS:...4
H
Data Transmission by Accessing Layer 2 Services
LAN bus
Receiver control program (Station 2 in the
example)
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
8-15