Siemens SIMATIC S5 Manual page 148

Data Transmission by Accessing Layer 2 Services
Request
Byte
0 com_class
FDL_Request=00
(request for layer 2 service)
1 user_id
User-definable ID
(echoed in a confirmation)
2 service_code
Type of service requested
3
irrelevant
4 Bits 4 to 7
service_class
Priority of
send frame
5 SAP No. of the receiver =
destination SAP
(hexadecimal)
6 rem_add_station
Address of the destination
station (hexadecimal)
7 rem_add_segment
Logical segment address;
always enter FF
(no other segments can be
currently addressed)
8 .
Send data
. .
(0 to 242 bytes)
249
Figure 8-4. Request, Confirmation and Indication Structures
You can store the requests and indications (up to 250 bytes each) in the flag area or data area.
In view of the relatively large maximum amount of data, however, we advise you to use only the
data area.
Section 8.5 describes the structure of requests, confirmations and indications for the special layer 2
services.
As a prerequisite for using layer 2 services, you must first
• specify an SAP number in DB1 ( section 8.3)
• define two status bytes in DB1 ( section 8.3)
• assign L2-SEND and L2-RECEIVE their parameters ( section 8.3, chapter 5)
• create DBs for the request, the confirmation and the indication ( from section 8.5 ff.).
8-8
Byte
0
FDL_Confirmation=01
H
(Confirmation from layer 2
firmware following request)
1
ID assigned in connection with
2
Type of service provided
3
OK or error message; indicates
success or failure of preceding
4 Bits 4 to 7
Bits 0 to 3
0 service_class
Priority of the
send message
5 SAP No. of the receiver =
6
Address of the destination
7 rem_add_segment
Logical segment address;
H
(no other segments can be
8 .
Receive data in connection with
. .
SRD service (0 to 242 bytes)
249
Confirmation
com_class
H
user_id
a request
service_code
link_status
request
Bits 0 to 3
0
destination SAP
(hexadecimal)
rem_add_station
station (hexadecimal)
is always FF
H
currently addressed)
S5-95U, SINEC L2
Indication
Byte
0 com_class
Indication=02
(indication that data have been
received)
1
irrelevant
2 service_code
Type of service provided
3
irrelevant
4 Bits 4 to
service_class
Priority of
receive frame
5 SAP No. of the sender =
source SAP
(hexadecimal)
6 rem_add_station
Address of source station
(hexadecimal)
7 rem_add_segment
Logical segment address;
is always FF
(no other segments can be
currently addressed)
8 .
Receive data
. .
(0 to 242 bytes)
249
EWA 4NEB 812 6112-02
H
3
Bits 0 to
0
H