In
logic, a
four-valued logic is used to model signal values in digital circuits: the four values are Z, X and the
boolean values 1 and 0. Z stands for
high impedance or
open circuit, while X stands for "unknown". There is also a 9-valued logic standard by the IEEE called
IEEE 1164.
There are other types of four value logic, such as
Belnap's four-valued
relevance logic: the possible values are 1) true, 2) false, 3) both true and false, and 4) neither true nor false. Belnap's logic is designed to cope with multiple information sources such that if only true is found then true is assigned, if only false is found then false is assigned, if some sources say true and others say false then both is assigned, and if no information is given by any information source then neither is assigned.
Applications
Electronics
Digital electronics theory supports
four distinct logic values (as defined in
VHDL's
std_logic):
- 1 or High, usually representing TRUE.
- 0 or Low, usually representing FALSE.
- X representing a "Conflict".
- U representing "Unassigned" or "Unknown".
- - representing "Don't Care".
- Z representing "high impedance", undriven line.
- H, L and W are other high-impedance values, the weak pull to "High", "Low" and "Don't Know" correspondingly.
The "X" value does not exist in real-world circuits, it is merely a placeholder used in simulators and for design purposes. Some...
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