Designing and Testing simultaneously  

The real power of LogSim is the model behind the drawing. Each time the user adds or deletes a logic element on the diagram, makes or deletes an input connection or changes the attributes of an element, LogSim updates the logic model in the background.
The behavior and the functions of this model are the same as of a real logic system, which will be built based on the diagram. This model can be activated any time in order to simulate the functions of the schematics. So the drawing on the screen is a "living" schematic, which can be functionally tested the same way as a PLC. 

This test gives even more information than a PLC test, because testing of a PLC shows only the states of the inputs and outputs, while the LogSim screen shows the states of all elements of the entire logic.

A number of powerful auxiliary functions help the user to trace and correct eventual errors: the functional input and output labels, the history display and the single step mode. Unnecessary to say that this makes the tracing and correcting of errors much easier and efficienter.

The example below shows the simulation of a simple logic with switches, pushbuttons, AND gate,a timer and a signal light:

When the user pushes the "Start" button on the toolbar, LogSim first carries out a consistency test. The number of connected input signals to each element is tested, and in case the number is not correct, an error message is issued. A second check on the presence of algebraic loops follows. If the structure of the system is correct, then the elements are sorted in such a way that the signal transmission takes place in the correct sequence. Subsequently the elements are set to the correct starting state and the cyclic scanning of the inputs and determining of the output states starts.

This mode is recognizable by the color change of elements and connecting lines. During the run the state of the input elements can be changed (like during the functional test of a PLC) and the effect can be followed throughout the logic. Different options are available for the efficient analysis of large systems. The name labels of all inputs and outputs can be displayed on the left and the right edge of the screen. The input labels can be used as push buttons or switches; the output labels show the state of the outputs. Elements can be grouped on the history display, which shows the state changes of the elements graphically. LogSim also can be run in Single Step mode, where the propagation of the state changes can be followed throughout the logic.