Siemens SIMATIC S7-400H System Manual page 456

Differences between fault-tolerant systems and standard systems
Function
Use of SFC 49 "LGC_GADR"
Call of SFC 51 "RDSYSST" with
SSL_ID=W#16#xy91
Web server
PROFINET CBA
IRT
Isochronous mode on PN
Tool changer
Fast Startup
Use of an external PN controller
See also
System and operating states of the S7–400H (Page 121)
456
Restriction of the fault-tolerant CPU
You are operating an S7-400H automation system in redundant
mode. If you declare the logical address of module of the switched
DP slave at the LADDR parameter and call SFC 49, the high byte
of the RACK parameter returns the DP master system ID of the
active channel. If there is no active channel, the function outputs
the ID of the DP master system belonging to the master CPU.
The data records of the SSL partial lists shown below cannot be
read with SFC 51 "RDSYSST":
SSL_ID=W#16#0091

SSL_ID=W#16#0191

SSL_ID=W#16#0291

SSL_ID=W#16#0391

SSL_ID=W#16#0991

SSL_ID=W#16#0E91

Not integrated
Not possible
Not possible
Not possible
Not possible
Not possible
Not possible
System Manual, 03/2012, A5E00267695-11
S7-400H