Development of a relay ladder logic programming/debugging/simulation software package with graphical user interface

Programmable Logic Controllers (PLC’s) are widely used in industry for Logic and Sequencing Control. Typically, PLC’s are programmed using Relay Ladder Logic, in which the programmer draws a circuit diagram schematic for a set of conceptual logic circuits with which to control physical equipment. The PLC then emulates the activity of these circuits. Many students experience difficulty when first exposed to Relay Ladder Logic programming. This difficulty arises due to many reasons, including the difficulty of imposing program execution flow control, and the parallel nature of Relay Ladder Logic program execution. The research described here addresses these issues. A software package (LadderCAD) was developed which overcomes equipment specificity issues, and which addresses instructional issues by employing two interpreters: 1) A forward interpreter which translates Relay Ladder Logic diagrams into System State Language (SSL), a textual description of the system state for which any given physical output will be energized, and 2) A backward interpreter which translates SSL files into Relay Ladder Logic diagrams. Using these two interpreters, students may more easily make the transition from textual programming languages to Relay Ladder Logic programming. A simulation utility was also developed which may be used by the student to run Ladder Logic programs on-screen for troubleshooting and verification purposes. This simulation package allows the user to monitor the status of all inputs and outputs simultaneously on a single screen --Abstract, page iii