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A d-latch is a circuit that stores the last value seen at its "data" input while its "enable" input was on.

Definition

value table

inputs			outputs
enabled	data	previous output	output
0	x	y	y
1	x	y	x

Connectors

• Enabled: if 1, store data, else do nothing.
• Data: input
• Output: stored value

Relation to Multiplexer (Logic Select Unit)

In theory, a single multiplexer (Logic Select Unit) can do this (selector input = enable, input 1 = output, input 2 = data, output = output). In practice, this is not possible as the game does not allow it to have its own output as input. However, several constructions using two or more circuits are possible.

Implementation Using Logic Select Unit and Logic Reader

D-latch using select unit and reader

This d-latch can store any value. It works in two steps:

1. a Logic Select Unit determines whether to store the last output or the "data" input using the "enabled" input. i.e.

     if enabled
     then store = data
     else store = output

2. a Logic Reader mirrors the value to be stored and feeds it back into the select unit so it can circulate (effectively being stored). This would be obsolete if the select unit could read its own output.

     output = store

Circuit setup

Unit name	Unit type	Connector / Setting	Source / Value
select 1	Logic Select Unit	select	enabled (input)
		0 (input 1)	reader 1
		1 (input 2)	data (input)
reader 1	Logic Reader	in	select 1
		var	setting
		out	output (output)