Siemens SIMATIC S5 Manual page 292

STEP 5 Programming
9.1.5 Automatic Synchronization in OB 1
Mode 99 is the first mode the IP 266 will accept. Only then can one of the modes
1 to 73 be started. Mode 99 should therefore be invoked automatically in OB 1.
The TOGGLE bit must always be set to "1" for this mode.
Note:
No status info is returned for mode 99. If this mode executes without error, bit 0
is set in byte 2 of the input message (PII).
9-4
IP 266
EWA 4NEB 812 6057-02