1.1 From flow code to machine behavior

In classical PLC programming you think in commands: If a signal is present, a line of code is executed; if a condition is true, an output is set. This approach worked for decades, but it has two central weaknesses:

1. The behavior of the machine is only indirectly recognizable. The program code describes, what is switched, but not, why. Even experts often need hours or days to reconstruct from thousands of lines of code how a machine actually operates.

2. The sequence is not formally described. A classical PLC can execute many branches, subprograms and side conditions simultaneously. This leads to unpredictable states, race conditions and hard-to-reproduce errors.

Selmo takes a different approach: Instead of executing commands, it describes behavior. No longer “if input X, then output Y”, but “the machine is in state Z, and in this state the following rules apply.”

This changes the direction of thinking: From programming (writing instructions) to modeling (describing behavior).