PROFIBUS Manual
At the interface of FDL user to FDL, data transmission services are processed across a service access point (SAP). So during message transmission the appropriate SAP must also be sent.
Example of an SD2 frame with SAPs:
SD2 |
LE |
LEr |
SD2 |
DA |
SA |
FC |
DSAP |
SSAP |
PDU |
FCS |
ED |
The source service access point (SSAP), which represents the access address of the local user to the FDL is transferred in the SSAP byte. The destination service access point (DSAP), which represents an access address from the remote user to the FDL, is transferred in the DSAP byte (compare also with diagram below).
SSAP and DSAP
This SAP is used for distinguishing which application and, in the case of the DP protocol, which service should be used.
The role of a SAP
An initial assignment of possible values for master and slave stations can be found in the following tables:
SAP: service access points of master
SAP |
Name |
Use |
NIL |
Data_Exchange |
DP MS0: cyclic data exchange |
0 (0x00) - 1 (0x01) |
|
FDL management |
other |
Free |
Freely assignable by FMS or other protocols |
50 (0x32) |
MS2 |
DP MS2: acyclic master class 2 |
51 (0x33) |
MS1 |
DP MS1: acyclic master class 1 |
54 (0x36) |
MM |
DP master to master connection |
62 (0x3E) |
MS0 |
DP MS0: slave handler per DP slave |
SAP: service access points of slave
SAP |
Name |
Data unit from master to slave |
Data unit from slave to master |
|---|---|---|---|
NIL |
Data_Exchange |
Output data |
Input data |
0 & 1 |
FDL Management |
|
|
2... |
free assignment |
|
|
≤48 (0x30) |
Communication |
DS_xxx_REQ, MS2_Abort_REQ |
DS_xxx_RES |
49 (0x31) |
Resource Manager |
MS2_Initiate_REQ |
Resource_Manager_REQ |
50 (0x32) |
Alarm_SAP |
DS_Alarm_ack |
|
51 (0x33) |
Server_SAP |
DS_Read_REQ, DS_Write_REQ |
DS_Read_RES, DS_Write_RES |
53 (0x35) |
Ext_User_Prm |
Ext. Parameter |
SC |
55 (0x37) |
Address |
SC |
|
56 (0x38) |
Rd_Inp |
Blank |
Input data |
57 (0x39) |
Rd_Outp |
Blank |
Output data |
58 (0x3A) |
Control |
--- |
|
59 (0x3B) |
Get_Cfg |
Blank |
Configuration |
60 (0x3C) |
Slave_Diagnosis |
Blank |
Diagnosis |
61 (0x3D) |
Parameter |
SC |
|
62 (0x3E) |
Configuration |
SC |
|
63 (0x3F) |
Broadcast |
|
|
