Shortcomings

Logisim's propagation algorithm is more than sophisticated enough for almost all educational purposes; but it is not sophisticated enough for industrial circuit design. In order from most damning to least damning, the shortcomings of Logisim's propagation technique include:

• Logisim cannot simulate subcircuits whose pins sometimes behave as inputs and sometimes behave as outputs. Components built using Java can have such pins, though: Within the built-in libraries, the Memory library's RAM circuit contains a D pin that acts both as an input and as an output.

• Logisim cuts off its simulation after a fixed number of iterations assuming that there is an oscillation error. Conceivably, a large circuit that does not oscillate could lead to trouble.

• Except for the issue of gate delays, Logisim does not particularly concern itself with timing issues. Its introduction of occassional randomness in component delays makes the situation somewhat more realistic, but it is still a very idealized version of reality.

• Logisim does nothing with respect to discriminating between voltage levels: A bit can be only on, off, unspecified, or error.

• There are additional shortcomings, too, that I have omitted because they are so obscure that if you were aware of them, it would be obvious that Logisim comes nowhere close to that level. As an extreme example, I have a friend who works for a major chip manufacturer, and his job is to worry about ``bubbles'' in chips' nanometer-wide wires growing and leading to random disconnection.

• Even beyond this, I am not a circuit design specialist; thus, there may well be errors in the propagation technique of which I am not aware. I welcome corrections from experts.

Next: User's Guide.