General > Frequently Asked Questions

I don't get ladder logic.

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Adam:
What would the ladder logic look like to create the movement of an X-Y table (with the use of two stepper motors) to create a parabolic function and a circle?

Myers:
I tend to think of my PLC's differently.  There is no real way to do what you want with ladder logic.  It's easier to accomplish with a function as an output in ladder logic.

Ladder logic deals primarily with solid state.  A device is either ON or OFF.  A switch is either OPEN or CLOSED.  Each device, say for example a light bulb, has a number of things that have to happen to turn on.  Switch 1 may need to be open, Switch 2 may need to be closed and either Switch 3 or Switch 4 must be closed.  It relatively easy in ladder logic to perform this.

Wher Tri-Logic comes in is that instead of turning on a light with all those switches, you can turn on a BASIC program that will run and perform your functions.

In your case, a function output would be controlled by inputs called "E-Stop Pressed" AND "Light Curtain Clear" AND "Start Button Pressed".  When all three of those events happened at the same time, a function (Output) would be run to pulse your steppers incrementing or decrementing your X and Y values to make your pattern.

The challenge is not really in the Ladder Logic, but in the BASIC function!

Mark Myers

support:
Chapter 8 in the TRiLOGI Programmer's Reference Guide has a short introductory course on Ladder Logic Programming. For those interested, please download the Chapter below:

  www.tri-plc.com/appnotes/Chapter8.pdf

support:
You can now go to the following web page to download a free PLC programming E-book written by Kevin Collins of Ireland Sligo Institute:

http://www.tri-plc.com/reqplcbook.htm

This book presents a number of ladder programs that are constructed using the TRiLOGI software and you can use our PLC simulator to test the program.

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