The Omron NS-CLK21 is an Option Card, Communications accessory designed for the NS Series of operator interfaces. It provides a dedicated communication link (CTL LINK) for networked data exchange between NS HMIs and compatible controllers on the same link.
The NS-CLK21 mounts inside an NS Series HMI enclosure, occupying the option-card slot. It is not a standalone module; verify that your specific NS HMI model supports this card and that the firmware revision is compatible with the CTL LINK protocol version used on your network.