ISA5.2Use of symbols

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By using the symbols designated as "basic," logic systems may be described with the use of only the most fundamental logic building blocks. The remaining symbols, not basic, are more comprehensive and enable logic systems to be diagrammed more concisely. Use of the non-basic symbols is optional.
A logic diagram may be more or less detailed depending on its intended use. The amount of detail in a logic diagram depends on the degree of refinement of the logic and on whether auxiliary, essentially non-logic, information is included.
As an example of refinement of detail: A logic system may have two opposing inputs, e.g., a command to open and a command to close, which do not normally exist simultaneously; the logic diagram may or may not go so far as to specify the outcome if both the commands were to exist at the same time. In addition, explanatory notes may be added to the diagram to record the logic rationale.
Non-logic information may also be added, if desired, e.g., reference document identification, tag numbers, terminal markings, etc.
In these ways, the diagram may provide the level of detail appropriate, for example, for communication between a designer of pneumatic circuits and a designer of electric circuits, or may provide a broad-view system-description for a plant manager.
The existence of a logic signal may correspond physically to either the existence or the nonexistence of an instrument signal, depending on the particular type of hardware system and the circuit design philosophy that are selected.* For example, a high-flow alarm may be chosen to be actuated by an electric switch whose contacts open on high flow; on the other hand, the high-flow alarm may be designed to be actuated by an electric switch whose contacts close on high flow. Thus, the high-flow condition may be represented physically by the absence of an electric signal or by the presence of the electric signal. The Standard does not attempt to relate the logic signal to an instrument signal of any specific kind.
It is recommended, for clarity, that a single time-function symbol, as appropriate, be used to represent each time function in its entirety. Though not incorrect, the representation of a complex or uncommon time function by using a time-function symbol in immediate sequence with a second time-function symbol or with a NOT symbol should be avoided (see Section 4.8).