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Data exchange between two safety cpu

Data exchange between two safety cpu

New Poster


A customer ask how many data it's possible exchange between two safety cpu,

In other word if there is a limit to variables transfering ?

thanks in advance



Community Moderator
Community Moderator

Hello Massimo,

I'm sorry for the delayed answer.

There are two ways of safe cross communication, as shown during the recent presentations:

  • SMMC (Safe Master to Master Communication)
    • It is based on Standard-Ethernet or EtherCAT Automation Protocol (connecting ctrlX SAFETY SAFEX-C controllers in different EtherCAT networks). In the variant based on Standard-Ethernet it needs no extra fieldbus. It is just UDP (TCP/IP) based.
    • It connects up to 4 controllers
    • Each controller can provide up to 2 byte (16 bit) of safe date
    • It is exchanged at the processing cycle of the safe PLC (4 ms with ctrlX SAFETY SAFEX-C)

SMMC via EtherCAT Automation ProtocolSMMC via EtherCAT Automation Protocol


SMMC via Standard EthernetSMMC via Standard Ethernet

SMMC via Standard Ethernet in combination with UplinkSMMC via Standard Ethernet in combination with Uplink


  • FSoE (Safety over EtherCAT) 
    • It is used to connect FSoE-Master with FSoE-Slaves. The later can be any kind of FSoE-Drives, -I/Os or other ctrlX SAFETY SAFEX-C controllers. It needs a running and correctly configured EtherCAT-Network.
    • An FSoE-Master supports connections to up to 32 FSoE-Slaves.
    • Each connection includes 12 safe input and 12 safe output bytes.
    • The data exchange is done at the cycle time of the EtherCAT network, but you have to consider
      • The basic cycle is the one of the safe PLC (see above)
      • Every FSoE-connection is going through the Standard EtherCAT-Master that needs to copy the incoming data in EtherCAT data frames to the very same for outgoing data. This is valid in principal for all connections from the FSoE-Master to the FSoE-Slave  and vice-versa.
      • In the standard case the PLC at the EtherCAT-Master is copying the incoming to the outgoing data frames (e.g. with ctrlX CORE). With higher integration this can be shifted into the EtherCAT-Master stack. This has no impact on the timing, since the ctrlX CORE PLC can use a fieldbus synchronous task for that action. This provides only 1 EtherCAT cycle delay for the outgoing data in the best case. Shifting it the the EtherCAT-Master stack is only an improvement in usability.

FSoE safe cross communication between ctrlX SAFETY controllers and other FSoE-SlavesFSoE safe cross communication between ctrlX SAFETY controllers and other FSoE-Slaves

Hope this helps. If you like this and it is a valid answer to your question, please mark it as a solution.

Best regards

Your ctrlX SAFETY team