A single-input timer called a non-retentive timer is used in some PLCs.
Energizing I:1/0 causes the timer to run for 4 seconds.
At the end of 4 seconds the output (DN) goes on. When the input is de-energized, the output goes off and the timer resets to 0.
If the input I:1/0 is turned off during the timing interval (for example, after 2.7 seconds), the timer resets to 0.
TON is the basic non-retentive timer in Allen-Bradley PLCs