Sending Data And Fetching Data From A Station (Srd Service) - Siemens SIMATIC S5 Manual

S5-95U, SINEC L2
8.9

Sending Data and Fetching Data from a Station (SRD Service)

The SRD (Send and Request Data with Reply) layer 2 service is used by an active station to send
data to an active or passive station and/or fetch data from an active or passive station.
Prerequisite for data transmission:
You must hold data requested by the sender ready for fetching with the RUP_SINGLE
( section 8.7) or RUP_MULTIPLE layer 2 service ( section 8.8)
Data transmission schematic ( Figure 8-14):
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 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 + net data requested by the receiver) with L2-
RECEIVE.
Sender control program (Station 1 in the
Request SRD SEND
Header with 8 bytes
SRD request
In example: DB33,
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
Data
In example: DB33,
DW 14 to DW 17
Figure 8-14. Sending and Fetching Data with the SRD Service
EWA 4NEB 812 6112-02
S5-95U
Request
(Data)
STBS:...2
H
Confirmation
H
(acknow-
ledgement
+data)
RECEIVE
STBS:...4
H
Data Transmission by Accessing Layer 2 Services
LAN Bus
Receiver control program (Station 2 in the
example)
SAP
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
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