Instruction manual

Timers
110
890USE18300 September 2003
Enable Input A delay-to-start timer block 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 timer finishes a timing cycle if it has already started it, but it does not
change the output—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.
Timer Trigger
Input
The trigger input is a set of Boolean 1s and 0s. The rising edge of the trigger input
starts a timing operation, and the falling edge of the trigger input causes the timer
accumulator to drop to 0.
The value of the trigger input is important for the output of the block. If the trigger
drops to 0 before the timer reaches the terminal count, the timer stops accumulating
and drops to 0. When this happens, the output never turns on. If the trigger remains
at 1 after the terminal count has been reached, the timer accumulator holds the
terminal count value and the output rises to 1.
The timer trigger input 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 timer is the second block in a nested reflex action (See Nesting Two Reflex
Blocks, p. 36), its trigger input may be configured as the output of the first reflex
block
Note: At start-up, make sure that the trigger input provides a 0 to the timer block.
If the trigger is 1 when the timer becomes enabled, the timer assumes that a rising-
edge transition has just taken place and starts accumulating counts immediately.
If the trigger input is 0 when the block is enabled, the timer waits for the next rising-
edge transition before it starts the time-unit accumulation.