Introduction to Applicative System Redundancy (ASR)

Available with AXC F 2152 and AXC F 3152

The Applicative System Redundancy (ASR) is realized by providing two redundant PLCs (First and Second) in the same network together with the responding I/O stations. The communication between the PLCs and the I/O stations is based on the PROFINET® RT protocol. 

ASR Overview


On both controllers (First  and Second PLC) the identical application program is in operation. One of the PLCs is the active controller (serves as Primary controller) and the other is the standby controller (serves as Backup controller). The Primary controller controls the process. The redundancy role (Primary or Backup) is specified by the application software. Therefore, the responding function block needs to be implemented and parameterized in the application program. If the Primary controller fails or some other user defined switch-over condition occurs, the Backup controller takes over the process control.

The use of function blocks for data synchronization allows selected data to be continuously synchronized between the Primary and the Backup controller during normal operation (as long as both controllers are available). By holding and processing identical application data, a seamless switch-over and a short take-over time is possible from the Primary to the Backup controller in case of malfunction of the Primary controller.

System requirements

The Applicative System Redundancy is based on the mechanism of the PROFINET® system redundancy. This PROFINET® standard describes several types of system redundancy. The PLCnext Technology control system supports PROFINET® system redundancy type S2 according to that PROFINET® specification.

PROFINET® IO devices used in such a system must support PROFINET® system redundancy type S2, including the RT_InputOnBackupAR_Supported option activated. This option ensures that the IO devices provide valid input data also for the backup controller.

PROFINET® IO devices that do not support this option cannot be integrated into the PROFINET® bus configuration when the PROFINET® system redundancy type S2 is activated.

The IO stations are directly integrated into a network ring structure via the Media Redundancy Protocol (MRP) by using two Ethernet switches. One switch must be configured as MRP manager and the other as MRP client. In such a network ring structure with Media Redundancy Protocol, a maximum network recovery time of 200 ms can be expected.

Hardware recommendation

  • First and second PLC (PROFINET IO controller):
    AXC F 2152 or AXC F 3152 with firmware version ≥ 2021.0 LTS
  • Ethernet switch MRP manager and MRP client:
    FL SWITCH 22xx with firmware version ≥ 3.00 + FL SD FLASH/MRM
  • IO stations (PROFINET IO device):
    AXL F BK PN TPS with firmware version ≥ 1.31 + Axioline F IO modules

Function blocks for ASR

From the PLCnext Store, download the ASR AXC F 2152 or the ASR AXC F 3152 function block library (free of charge) and import it into PLCnext Engineer on your computer.



•  Web browser recommendation: Chrome/Edge 88 or newer, Firefox ESR 90 or neweror Safari • 
• Published/reviewed: 2022-09-14 • Revision 046 •