What are the differences between Pulse Timer (SP) and Delay Timer (SD) in the industrial PLC domain?
In industrial settings, there is often a need to handle time-related logic, such as the delayed opening of valves or the delayed shutdown of motors. The processing of such logic requires the use of timers in the PLC (Programmable Logic Controller) resources.
Siemens' Step7/Portal software series provides a variety of timer types, allowing users to choose the appropriate timer based on on-site requirements. In today's article, we will discuss two timer types: the Pulse Timer (SP) and the Delay Timer (SD).
Every timer has three signals: the start signal (Start Input), reset signal (Reset Input), and output signal (Output). For the sake of clarity in this article, let's assume I 2.1 is the start signal for the timer, I 2.2 is the reset signal, and Q4.0 is the output signal of the timer.
Firstly, let's take a look at the Pulse Timer (Start Pulse Timer),
From the timing diagram of the Pulse Timer (SP), we can observe the following information:
When the value of the start signal I 2.1 transitions from 0 to 1 (rising edge), the timer begins counting.
Once the timer starts, if the start signal I 2.1 remains 1 and the reset signal I 2.2 is 0, the timer's output is 1 (Q4.0=1).
While the reset signal I 2.2 remains 0 and the start signal I 2.1 stays at 1, when the set time 't' elapses, the timer's output transitions from 1 to 0 (Q4.0 changes from 1 to 0).
If the reset signal I 2.2 remains 0 and the start signal I 2.1 transitions from 1 to 0 before the set time 't' elapses, the timer stops counting, and the output signal changes from 1 to 0 (Q4.0 changes from 1 to 0).
When the reset signal I 2.2 transitions from 0 to 1, the timer stops counting, and the output signal changes from 1 to 0 (Q4.0 changes from 1 to 0).
While the reset signal I 2.2 remains 1, the timer's output remains 0 (Q4.0=0).
The Pulse Timer (SP) is suitable for situations requiring delayed shutdown. For instance, in the field, after closing a water valve, if we want the remote water pump to continue running for a certain period to maintain pressure in the pipeline, a Pulse Timer can be used.
Now, let's take a look at the timing diagram of the Delay Timer (Start Delay Timer, abbreviated as SD):
From the timing diagram of the Delay Timer (SD), we can observe the following information:
When the value of the start signal I 2.1 transitions from 0 to 1 (rising edge), the timer begins counting.
Once the timer starts, if the start signal I 2.1 remains 1 and the reset signal I 2.2 is 0, when the set time 't' elapses, the timer's output transitions from 0 to 1 (Q4.0 changes from 0 to 1).
After the timer's output transitions from 0 to 1, if the start signal I 2.1 remains 1, the output signal remains 1 (Q 4.0=1). If the start signal I 2.1 transitions from 1 to 0 (falling edge), the output signal becomes 0 (Q4.0=0).
After the timer starts, if, before the set time 't' elapses, the start signal I 2.1 transitions from 1 to 0 (falling edge), the output signal remains 0. On the next rising edge of the start signal, the timer restarts counting.
When the timer's output is 1 (Q 4.0=1), if the reset signal I 2.2 transitions from 0 to 1, the timer's output becomes 0 (Q 4.0=0).
If the reset signal I 2.2 remains 1, the timer's output remains 0 (Q 4.0=0).
The Delay Timer (SD) is suitable for situations requiring delayed opening/starting. For example, in some on-site alarm scenarios, to prevent signal jitter, when an alarm signal triggers (transition from 0 to 1), there is often a need to introduce a time delay. If the alarm signal persists after a certain duration, the alarm function is then triggered.
Remember this: The output of the Pulse Timer (SP) changes from 1 to 0; the output of the Delay Timer (SD) changes from 0 to 1. This is their fundamental difference.