PROFINET "S2 redundancy" denotes an architecture in which two IO Controllers (one primary, one standby) are connected to the same IO Device, with automatic failover in case of primary failure and without loss of communication.

S2 means "Single Device, dual Controller", as found typically with redundant PLC pairs such as the Siemens S7-1500H or the ABB AC800M HI. The Device maintains two ARs in parallel, one with the primary and one with the backup Controller; on primary failure, the device fails over to the backup AR in less than 100 ms. The full PROFINET redundancy family is: S1 (one device, one controller), S2 (one device, two controllers - hot standby), R1 (two devices, one controller - device redundancy) and R2 (two devices, two controllers - full redundancy).

Verify in the device GSDML that S2 is supported before promising hot-standby behaviour; many older PROFINET devices only support S1 and will simply drop communication on Controller failover.