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
