QUESTION:
Which parameters do I have to assign to the system functions SFC103 "DP_TOPOL" and SFC59 "RD_REC" to be able to identify and read out the topology data of a diagnostics repeater (DR)?

ANSWER:
The system function SFC103 "DP_TOPOL" triggers identifying of the topology for a selected DP master system. In order to be able to use this system function your CPU has to meet certain requirements. These are given in detail in Entry ID: 17452619.

An SFC103 call addresses all diagnostics repeaters on a DP master system.

Parameter	Meaning	Data type	I/O
REQ	Trigger identifying of topology	BOOL	I
R	Abort identifying of topology	BOOL	I
DP_ID	ID of the DP master system whose topology is to be identified (see Fig. 1)	INT	I
RET_VAL	Return value of the job status	INT	O
BUSY	Identifying of topology not yet finished	BOOL	O
DPR	PROFIBUS address of the diagnostics repeater reporting the error	BYTE	O
DPRI	Measuring segment of the diagnostics repeater reporting the error	BYTE	O

Table 1: Calling parameters of the SFC103 "DP_TOPOL" - identifying of topology

When SFC59 "RD_REC" is called, the data stored in a diagnostics repeater is read out. The volume of the data that can be read out by a SFC59 call is limited to 240 bytes. The diagnostics repeater however provides 675 bytes of data only for the topology. The topology data in the diagnostics repeater is therefore stored in 4 different data records which are only 170 or 165 bytes in size. Thus with four calls of the SFC59 all the topology data of a diagnostics repeater can be read out.

Parameter	Meaning	Data type	I/O
REQ	Request to read a data record	BOOL	I
IOID	ID of the address area - the DR is configured as peripheral input (PI) (B#16#54)	BYTE	I
LADDR	Logical address of the module - the diagnostics address is to be used with the DR (see Fig. 1)	WORD	I
RECNUM	Data record number (valid values for the identifying of topology data 32h...35h for the data records 50...53)	BYTE	I
RET_VAL	If an error occurs during processing of the function, the return value contains an error code. In addition: length of the data record actually transferred in bytes (possible values: 165 or 170), if the target area is greater than the data record transferred and if no error occurs during the transfer.	INT	O
BUSY	The read procedure has not yet finished	BOOL	O
RECORD	Target area for the data record read	BYTE	O

Table 2: Parameters of the SFC59 "RD_REC" - read data record

Fig. 1: Parameters from the HW Config required for the SFC103 and SFC59 call ( 56 KB )  

Recommendation for Programming: 
The structure of four data records of the topology table is identical. If you create an UDT, in which the structure is determined uniquely, then you can use this UDT several times when specifying the structure of the required data block. This saves you creating the data structure time and again for every data block. In case of changes only the UDT is to be changed.

Note:
More information on this topic is available in the STEP 7 Online Help or in section 13.7 "Identifying the Bus Topology in a DP Master System with SFC 103 DP_TOPOL" of the manual "SIMATIC System Software for S7-300/S7-400 System and Standard Functions". The manual is available in Entry ID: 1214574.