X.21 is an Out-dated interface.
Note that there were a number of standards in the 'X' series.

X-Bus: An Obsolete four wire bus based on RS-485. Two wires for data, and two wires for power.

X-by-Wire Bus; Is a type or class of vehicle Bus.

XGA [Extended Graphics Array]: IBM introduced the XGA video interface as a successor to its 8514/A display in 1990.

XIO: Is an interface used on some SGI computers to interface between ASIC processors and memory controllers, in the mid 2000's}

XGMII (10 Gigabit Media Independent Interface. Uses SSTL_2 interface levels. provides an optional interconnection between the Media Access Control (MAC) sublayer and the Physical layer (PHY) of 10 Gigabit Ethernet.

XLR; Is a connector used with audio and video connections, per IEC 61076-2-103.

XSBI (10-Gb Sixteen Bit Interface}

XTX (eXtended Technology ETX) with a form factor of: 95mm x 111.6mm (5.75 x 8 inches). XTX is an Embedded computer board standard, an extension of the ETX board. XTX is a board standard, not a bus standard. XTX removes the ISA interface on the X2 connector [used on ETX] and replaces it with PCIe, SATA, and USB interfaces.

XAUI Ten Attachment Unit Interface.

An alphabetic listing of released interface buses. Primarily the links point to pages that describe the physical and electrical interfaces. Interface Bus protocols are not addressed.