As product lifecycles become shorter, factories are pushed to develop small batches of many different products. The highly flexible control systems has become a necessity. The majority of existing automated industrial systems are controlled by programmable logic controllers(PLCs). In most cases, the control programs for PLCs are developed based on ladder diagrams(LDs). However, it is difficult to debug and maintain those LDs because the synthesis of LD itself mainly depends on the experience of the industrial engineer via trial-and-error methods. Hence, in this paper, we propose a discrete event model conversion algorithm for systematic analysis of LDs. The proposed discrete event model conversion algorithm is illustrated by an example of a conveyor system.