This information is part of the Modelica Standard Library maintained by the Modelica Association.

Description in VHDL is given by http://www.cs.sfu.ca/~ggbaker/reference/std_logic/src/std_logic_entities.vhd

Truth Table

DataIn	Enable	DataOut
*	U	U
*	X	UX
*	0	DataIn
*	1	Z
*	Z	UX
*	W	UX
*	L	DataIn
*	H	Z
*	-	UX

UX: if dataIn == U then U else X

tHL	Value: 0 Type: Time (s) Description: High->Low delay
tLH	Value: 0 Type: Time (s) Description: Low->High delay