EtherCAT periphery coupler unit (EPU) is the locomotive of the train built from EtherCAT IO modules. It can be connected directly to STP cabling EtherCAT network. It has got 2 pieces of Ethernet (RJ45) connectors. One is the input and the other one is the output. The output of previous EtherCAT periphery connects to input, and output goes forward to the INPUT of the next EtherCAT periphery. We connect EtherCat units with STP cable (Shielded-Twisted-Pairs) and we use RJ45 connectors. The task of this unit is coupling the high variety of IO modules to the EtherCAT bus. Besides of this it provides the power supply for the IO modules as well. The total current value determines the number of connectable units, which cannot be higher than the output current of the EPU unit. The ready state of hardware and software units of each part of the whole EtherCAT network has shown in two relay outputs (Machine-On1 and Machine-On2) in each EPU units for better safety. If the ready status is reached, both relays are ON, and the output contacts are closed. If the status is not ready for operation, relays will open the contacts.