A simulator for use in programmable controller ladder logic testing is developed. A simulation language, based on SIMSCRIPT 11.5, to model ladder logic run physical systems is also designed. This System Description Language (SDL) handles simulations of the physical systems and the corresponding ladder-system interactions via the usage of specially designed constructs. The simulation package uses as input an SDL program file describing the system to be simulated, and a ladder file containing the ladder written to control this system. The simulation processor generates the actual simulation from the description contained in the SDL program file. A ladder scanning procedure approximates the actual programmable controller scan as closely as possible. The simulator also incorporates the ability for the user to dynamically interact with, and control the simulation by manipulating ladder inputs from the keyboard. A rolling timing diagram display of a maximum of 7 elements can be created and continuously updated for viewing purposes. System simulations of functionally different manufacturing systems are created and run with their respective ladders on the simulator. Different ladders for the same system are compared for evaluating the performance of the control logic of each ladder. These comparisons are based on the viewing of timing diagrams generated by the ladders. Thus, an off-line ladder logic debugging environment is created. / Master of Science