schoolphysicsImage: schoolphysicsIn electronics a NOT gate is more commonly called an inverter. The circle on the symbol is called a bubble and is used in logic diagrams to indicate a logic negation between the external logic state and the internal logic state (1 to 0 or vice versa). Logic gate - Wikipedia Is this answer helpful?Thanks!Give more feedbackThanks!How can it be improved?How can the answer be improved?Tell us how Ladder Logic Symbols - All PLC Diagram Symbols Jun 28, 2015The ladder diagram graphical programming language is standardized by the PLCopen organization, and thereby the symbols used in ladder diagrams. Since ladder logic is a graphical programming language, the PLC programs written in ladder logic are a combination of ladder logic symbols.[PDF] Overview of IEEE Standard 91-1984 2 Symbol Composition. A symbol comprises an outline or a combination of outlines together with one or more qualifying symbols. The shape of the symbols is not significant. As shown in Figure 1, general qualifying symbols are used to tell exactly what logical operation is performed by the elements.