Comparison of Logic Languages

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ClearSCADA currently has four IEC 6-1131 logic languages available to users. Each language behaves differently and each has its own benefits and drawbacks, the following table lists some of these may be used as a simple guide in helping to determine which language to use for particular situations in ClearSCADA.

Please note that some of information described below could be considered subjective and as such the list below is not a hard and fast rule of usage. It should be considered as only a guide to help understand the relative benefits of the languages available

Language	Benefits	Drawbacks
Structured Text	Highly flexible in both operation and configuration

Can issue methods
Can read and write to internal data via SQL
Can access raw and processed historic data
Allows parameters (VAR INPUT) to be passed in
Supports indirect variables to dynamically reference database objects or SQL parameters (using VAR INPUT parameters)
Can write to local files and COM ports (but cannot read from either)
Arrays supported using the VECTOR keyword | Requires prior coding experience to use effectively, if not the learning curve can be steep
Can be difficult to understand the execution cycles
Debugging is text based
Timing functions (e.g. TON and TOF) not as simple as in other languages (see Wait (Sleep) in Structured Text) |

Function Block Diagrams

Simple drag and drop interface Debugging is graphical making on site service easier Good to start with to learn the functions available

Cannot issue methods or access data via SQL queries

Can be slower to write than ST when only simple functions are performed |

Sequential Function Charts	Excellent program flow control allowing for loops and pauses Steps make the control of program flow simpler Debugging is partly graphical	Cannot access data using SQL Can be difficult to understand the program flow when new to SFCs
Ladder Logic	In appearance, very similar to existing (AB/Omron/Mitsubishi etc) PLC ladder logic

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