Instruction manual

Digital Latches
156
890USE18300 September 2003
Low-level Digital D-latch Block
Summary A low-level digital D-latch block produces a latched output when the block’s trigger
is 0 and an unlatched output when the trigger is 1. The output is a Boolean 1 or 0.
You may invert the value of the output.
Structure of a
Low-level Digital
D-latch Block
A block diagram for a standard low-level digital D-latch is shown below:
standard output latched to a specific operational input value when the trigger = 0; unlatched
and equal to the current operational input when the trigger = 1
inverted output latched to the inversion of a specific operational input value when the
trigger = 0; unlatched and equal to the inverse of the current operational input when the
trigger = 1
The latch block has three inputs—an enable input, a latch trigger and an operational
input. The enable input turns the block on or off. The latch trigger causes the block
to latch onto or unlatch from a digital value. The operational input is a stream of
Boolean values on which the block operates.
When a standard output is unlatched, it will echoes the current operational input.
When an inverted output is unlatched, it will echoes the inverse of the current
operational input.
When a standard output is latched, it holds the value of the operational input when
the latch trigger drops from 1 to 0. When an inverted output is latched, it will hold the
inverse of the value of the operational input when the trigger drops from 1 to 0.
Enable Input A low-level digital D-latch can be enabled either by a Boolean 1 or an always
enabled constant. It can be disabled by a Boolean 0 or an always disabled constant.
If the enable input is a Boolean, it may be produced by:
a digital input from a module on the island
a digital output from the virtual module (See The Virtual Module, p. 28)
an output on the action module (See Using the Action Module as an Input to a
Block, p. 32) written to by the fieldbus master
When the enable input is a Boolean 0 or an always disabled constant, the block is
disabled—the action does not execute and the output is frozen in the state it was in
when the block became disabled. The block continues to process inputs but does
not act on them. If the block becomes enabled, it immediately begins acting on the
latest set of inputs received.
enable
latch trigger
low-level digital D-latch
operational input
output