Bistable Function Blocks

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Introduction

A bistable function block maintains its output in one of two stable states True and False. The output can be set or reset by applying a True signal to the Set or Reset inputs.

Two standard function bistable blocks are supported: SR and RS.

SR - Set Dominant Bistable

ParameterTypeDirectionDescription
S1BOOLInputSets the output Q1.
RBOOLInputResets the output Q1.
Q1BOOLOutputThe output value.

The Set Dominant Bistable is logically equivalent to:

Function_Block SR
        Var_Input S1, R : BOOL; End_Var
        Var_Output Q1 : BOOL; End_Var

        Q1 := S1 OR (Q1 AND NOT S);
    End_Function_Block

Standards Compliance

Table 34.1 - IEC 61131-3 Second Edition.

Table 43.1a - IEC 61131-3 Third Edition.

RS - Reset Dominant Bistable

ParameterTypeDirectionDescription
SBOOLInputSets the output Q1.
R1BOOLInputResets the output Q1.
Q1BOOLOutputThe output value.

The Reset Dominant Bistable is logically equivalent to:

Function_Block RS
        Var_Input S, R1 : BOOL; End_Var
        Var_Output Q1 : BOOL; End_Var

        Q1 := (Q1 OR S) AND NOT R1;
    End_Function_Block

Standards Compliance

Table 34.2 - IEC 61131-3 Second Edition.

Table 43.2a - IEC 61131-3 Third Edition.

Further Information

Standard Function Blocks

To learn about other standard function blocks.

Elementary Data Types

To learn about the different types supported by Fernhill SCADA.

Common Elements

To learn about other IEC 61131-3 Common Elements.

Glossary

For the meaning of terms used in Fernhill SCADA.