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.
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:
-
The AND block B006 operates as the R_TRIG.
-
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
Related information
Please follow these links to related and useful information:
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.