In some cases such as this, there is no way to make the diagram reflect all PLC software programs.

I have mentioned before that my goal for this series is to use a generic language to describe my ladder logic diagrams so that they might be used for any platform. The circuit above introduces a new element that I haven’t covered in previous ladder logic posts, the one-shot. Another might be that maintained hardware buttons generally cost more than momentary ones. Since maintained pushbuttons are already available in both hardware and HMI versions, why would you want to use a circuit like this to turn a momentary button into a maintained one? One important reason is that you can use software to reset all buttons to the off state under certain conditions, such as E-Stops or start-up. A toggling or maintained button on the other hand changes from ON to OFF or vice-versa every time the button is pressed, maintaining its condition until pressed again. Momentary pushbuttons, as opposed to maintained pushbuttons, are only in the ON state as long as you hold your finger on the button. The generic pushbutton, IN 2.0, might be a hardware (momentary pushbutton) or software (HMI momentary PB) button. It replicates a toggling pushbutton that may be either hardware or software based. The simple logic above performs an important function that is often needed in a PLC program. They may not seem advanced to everyone who reads this blog, but these 300 level topics will be things that as far as I know aren’t really covered anywhere else. By advanced, I mean that rather than covering basic information on Ladder Logic and program structure like I did in my 100 Series, or general programming routines as I did in the 200 Series, I am going to cover some of the “tricks” and techniques that are used in PLC programming.

Today’s post is the first “advanced” topic I have covered in this blog.

Allen bradley ladder logic program