Instruction manual

Timers

890USE18300 September 2003 111

Timer Reset

Input

The reset input is essentially a timer override mechanism. It may be a Boolean 1 or

0. The timer is operational when the reset value is 1; it does not operate when the

reset value is 0.

The reset 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

The following timing diagram shows how the value of the reset input effects the

output from the timer block:

At the beginning of the timing sequence, when the reset input is 1, the standard

output is 0 (the inverted output is 1) while the timer is accumulating. The standard

output rises to 1 (the inverted output drops to 0) when the terminal count (TC) is

reached. When the trigger drops to 0, the timer and the standard output drop to 0 (or

the inverted output rises to 1).

The second time the trigger input rises to 1, the timer begins to accumulate again.

But before TC is reached the second time, the reset input drops to 0, thereby

resetting the timer. The standard output remains at 0 (or the inverted output remains

at 1) during this second timing sequence.

When the reset input rises back to 1, the timer begins to accumulate again starting

at 0. The reason that the reset input is able to restart the timer is because the timer

trigger input is 1 when the reset rises to 1. Once TC has been reached, the standard

output rises to 1 again and stays there (or the inverted output drops to 0 and stays

there) as long as both the trigger input and the reset input hold the terminal count.

timer trigger

timer

output

reset

output

inverted