Instruction manual

Digital Latches

150

890USE18300 September 2003

Operational

Input

The operational input is a stream of Boolean 1s and 0s that is latched by the falling

edge of the latch trigger. It may be produced by:

a constant

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

if the latch is the second block in a nested reflex action (See Nesting Two Reflex

Blocks, p. 36), the latch trigger input may be the output from the first reflex block

The following timing diagram shows how the value of the trigger effects the output

of the latch action:

At the beginning of the sequence, the operational input is high when the latch trigger

falls from 1 to 0. The standard output is latched at 1 (the inverted output is latched

at 0).

The output value remains latched until the latch trigger falls from 1 to 0 a second

time. At that moment, the operational input is low. The standard output is latched at

0 (the inverted output is latched at 1).

When the latch trigger falls from 1 to 0 the third time, the operational input is high.

The standard output is latched again at 1 (the inverted output is latched at 0).

operational input

latch_trigger

output

inverted