1.ET-121 SCU, Site Controller Unit acts as a transmission inspector. ET-121enables two different mode of devices to communicate with each other. It interchanges signals between the two devices. This action leads system to a much more stable working mode. In case of main server failure the SCU can still independently process various tasks. Apart from its ability to translate communication signals, it can also function as a temporary storage. SCU can store up to 11400 files.

ET-121 provide 3 types of communication methods for users to choose one from the following: RS-232, RS-422optic-fibre isolated communication and RS-485 mutual communication. This controller provide two sets of ports: RS-232, RS-422 port and DDC 9000 port for users. When using these ports, there will simultaneously be two sets of communication leds to show their communication status.