Ladder Logic closely resembles electrical ladder diagrams 1.A Ladder logic program exists only in the PLC’s software. 2.In ladder Logic it is not the actual flow of current thru circuits. 3.In electrical diagram the devices are described as open or closed(ON or OFF), where as in Ladder Logic, 4.instructions are either TRUE or FALSE. 5.In Ladder Logic Program must contain at least one control instruction (output) and usually contains one or more conditions (inputs).