Instructional Algorithms Enhance Student Understanding Of Plc Ladder Logic Programming

This paper presents two techniques that helps students transition from relay ladder logic concepts to programmable logic controller (PLC) ladder logic programming. The first technique presents a very structured algorithmic process for the selection of PLC ladder logic input contact configurations for a given control problem. The second technique describes the use of ladder logic building blocks for the commonly used ladder logic instructions in the development of larger PLC ladder logic programs. Students, familiar with relay ladder logic (RLL) control, know that input switch and sensor contacts are wired into the relay logic rung to provide power flow to the output field device, which is wired into the output side of the ladder. If the normally open (NO) contacts of the switch are used in RLL, then the contact symbol for input switch is a NO contact symbol. The field device wired into the output side of the rung is energized or de-energized based on the state of the input contacts. PLC ladder logic inputs are logical representations of memory bits in the input register. Selection of the correct input instruction, examine if closed or examine if open, depends on: the type input field device contact used, NO or NC; the state of the input, active or not active; and the desired state of the output field device, energized or not energized. The selection table process and algorithm presented in the paper leads students to the correct PLC ladder logic input choice. The process also helps them understand the relationship between the physical field device input and output hardware, and the software logic represented in the PLC ladder logic program. As students incorporate PLC ladder logic instructions into a software solution for a sequential control problem, they are faced with PLC instructions that support two or more output options. Timers, for example, have three output options depending on the type of control required by the sequential control problem. The paper describes how instruction options are structured into multi-rung program building blocks with specific control functions. Students are better able to select an instruction, and the instruction output option most appropriate for a control requirement, when they understand the operation of smaller building blocks. Large ladder logic programs are just combinations of these instruction building blocks. Use of both of these student learning techniques in teaching PLC programming has demonstrated their effectiveness in moving students from relay ladder logic to an understanding or PLC ladder logic programming.