Text Selectors

Introduction

Many SCADA applications use data provided in text form. For example:

A weather report provided in Java Script Object Notation (JSON).
Energy export prices in Comma Separated Variables (CSV) format.
A plain text file containing a list of products allowed in a storage silo.

A Text Selector is a way of locating a field or value within a text document. For example the Text Selector Line(5) returns line number 5 from a plain text document.

Fernhill SCADA uses Text Selectors in these areas:

As part of the IOItemName of an I/O Data Tag when associated with an MQTT Connection. The Text Selector describes which part of a topic to map to the I/O Data Tag. See MQTT I/O Item Name for more information.
In the Selector parameter of the TextDocGetValue() function. The Text Document Data Object provides a way for IEC 61131-3 Code to read and interpret text documents.

Text Selector Syntax

A Text Selector uses this syntax:

Read-Transform ::=
    { JSON-Read-Transform | CSV-Read-Transform | Line-Read-Transform }

JSON-Read-Transform ::=
    'JSONata(' JSONata-Expr ')'

CSV-Read-Transform ::=
    'CSV(' Cell-Reference ')'

Line-Read-Transform ::=
    'Line(' Line-Number ')'

Where:

'JSONata(' ... ')' indicates the contents of the parentheses is a JSONata query.
JSONata-Expr is a JSONata Expression to select and transform data from a topic value formatted using JSON.
'CSV(' ... ')' indicates the contents of the parentheses is a cell reference.
Cell-Reference is a spreadsheet style cell reference to select a row and column. For example A1 or D50
Line-Number is a 1-based line number indicating which text line to return.

Examples

Read the first line of a plain text document:
```
Line(1)
```
Read row 2 and column 3 of a CSV document:
```
CSV(C2)
```
Read an array element of a JSON formatted document:
```
JSONata(A[4])
```

Further Information

Fernhill SCADA Concepts

To learn about other Fernhill SCADA Concepts.

Glossary

For the meaning of terms used in Fernhill SCADA.