If you are familiar with Programmable Logic Controllers (PLCs), you may expect to find rising-edge and fallin- edge trigger blocks in the Siemens LOGO! PLC.
By definition, the edge trigger blocks monitor a signal. Upon detection of a 0 to 1 transition, the rising-edge trigger (R_TRIG) function will output a 1 for a single PLC cycle. Likewise, the falling-edge function (F_TRIG) will output a 1 upon detection of a 1 to 0 transition. These are both considered one-shot triggers as they only respond to the edge of the monitored signal. The R_TRIG and F_TRIG function blocks respond once and only once, unlike edge-sensitive logic which will hold an output value for as long as the input evaluates to true.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Program It → PLC → Siemens LOGO!
Difficulty: 
Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 09 Mar 2026
Use the Edge-Sensitive AND gate as F_TRIG
The Siemens LOGO! Soft Comfort software V8 does not have an explicit R_TRIG or F_TRIG function blocks. However, it does include an edge-sensitive AND gate that will function as a one-shot, as shown in Figure 1.
As wired, the AND gates will produce a single cycle pulse upon detection of a change in input I1:
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The AND block B006 operates as the R_TRIG.
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Inverting block B005 along with B007 perform the F_TRIG operation.
Figure 1: Siemens LOGO! R_TRIG and F_TRIG functionality is provided via edge-sensitive AND gates.
Tech Tip: A PLC cycle time (also known as scan time) is the time it takes to read the input, perform the user’s program, write the outputs, and perform the necessary background processing. This time may vary depending on the length of the user’s program. It is also dependent on the number of expansion modules, and background networking such as Modbus over Ethernet.
Generally, the cycle time is fast relative to the machine being controlled. However, additional hardware and software complexity can slow down the PLC. In some cases, the PLC is no longer able to meet the real-time requirements. In those situations, a high-performance PLC equipped with high-speed interrupts may be required. It may also be useful to split the task across several PLCs.
Best wishes,
APDahlen
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.