Introduction

Configuring dozens of tags by hand — one address at a time, one index at a time — is where time is lost and numbers get mistyped. For that, several numeric properties of PascalSCADA components accept, in the Object Inspector, an expression instead of a number: you type memaddress+2, Tag*4 or selindex*30 and the IDE computes the value. Combined with Lazarus’ multiple selection, this configures a hundred components in one keystroke.

It is a design-time feature: the expression is evaluated when you press Enter and what gets stored in the .lfm is the resulting number. Nothing changes at run time — for values computed at run time, use TNumericExprTag.

The feature was announced in the post Expressions on all properties of base tags; the video below shows it in use:

How to type it

In the Object Inspector, in the property field, type the expression and press Enter. Two forms:

You type What happens
100 A normal value.
10*8+3 Absolute expression: the property receives the result (83).
+2, -10, *2, /2 Relative expression: it starts with an operator, and the property’s current value is put in front. +2 on a MemAddress = 40 gives 42; *2 gives 80.
memaddress+2 Expression with variables (each property’s list is below).

The result is always an integer: divisions are truncated (7/2 → 3). With a multiple selection the expression is evaluated once for each selected component, with that component’s variables — which is what lets memaddress+100 shift ten tags at once, each from its own address.

Syntax

The evaluator is Free Pascal’s TFPExpressionParser, with the math functions enabled:

  • operators: + - * / ^ (power), mod, parentheses;
  • comparison and logic: = <> < > <= >=, and, or, not;
  • if(condition, ifTrue, ifFalse) and case(selector, v1, r1, v2, r2, …, default);
  • functions: abs, sqr, sqrt, round, trunc, int, frac, exp, ln, log, sin, cos, arctan, pi;
  • variable names are case-insensitive (MemAddress and memaddress are the same).

There is no div: use trunc(a/b) (or just a/b, already truncated when stored).

Where it works and which variables exist

Tag addressing properties — on TPLCTagNumber, TPLCBlock, TPLCStruct and TPLCString, the properties PLCRack, PLCSlot, PLCStation, MemFile_DB, MemAddress, MemSubElement, MemReadFunction, MemWriteFunction and, on blocks and structures, Size. Available variables: the other properties of that list (plcrack, plcslot, plcstation, memfile_db, memaddress, memsubelement, memreadfunction, memwritefunction) and Tag (the component’s Tag property, a free integer). The property being edited is left out, to avoid a circular reference.

Index — on TPLCBlockElement, TPLCStructItem and TPLCStructString. Variable: Tag. (On TPLCBlockElement the field’s drop-down also offers the block’s valid indexes.)

Position and size of any control — Left, Top, Width and Height of every TControl on the form, not just PascalSCADA controls. Variables: left, top, width, height, tag and selindex.

Tag — the Tag property of any TComponent. Variables: the same position ones (left… when it is a control) and selindex.

selindex is the component’s index within the current selection (0 for the first selected, 1 for the second…). It is the variable that turns a multiple selection into automatic numbering.

Recipes

All start from a multiple selection on the form or datamodule (Shift+click, or dragging a frame).

Number the elements of a block. Select the 16 hand-made TPLCBlockElements in the desired order:

  1. in Tag, type selindex → each gets 0, 1, 2… 15;
  2. in Index, type Tag → Index = 0, 1, 2… 15.

The same goes for TPLCStructItem with regular offsets: Index = Tag*2 for INT fields, Tag*4 for REAL.

Consecutive tags. Ten TPLCTagNumbers that must point to D100, D101, … D109: select the ten, Tag = selindex, then MemAddress = 100+Tag.

Shift a set of addresses. The DB moved and every tag must advance 50 bytes: select them and type +50 in MemAddress. Changed DB: MemFile_DB = 12 on the whole selection.

Copied struct. You copied the items of a TPLCStruct to represent a second device whose structure sits 64 bytes further: select the copied items and type +64 in Index.

Write function consistent with the read one. On Modbus tags, MemWriteFunction = if(memreadfunction=1, 15, 16) picks the right block write function for each tag, according to whether it reads coils or registers.

Align controls. Select ten THMILabels and type selindex*24+8 in Top and 16 in Left: a column with a 24 px pitch. A 4×4 button grid: Left = (selindex mod 4)*90, Top = trunc(selindex/4)*40.

Stack controls of different sizes. Top = selindex*(height+4) only works when all have the same height; for different heights, align in two passes or use Lazarus’ Anchor Editor.

Limits
  • Object Inspector only: not from code, nor in properties edited in other dialogs (Tag Builder, collection editors).
  • Variables read the current value of the other selected components; an expression depending on a property you are still going to change must be typed after it.
  • Only the integer properties listed above. RefreshTime, MinValue, the zones’ Value1 and others do not accept expressions (for now — the list is in src/scada_dsng/scadareg.pas and src/hmi/hmiregister.pas, and extending it means registering one more TTagAddressPropertyEditor).
  • A syntax error is reported by the IDE and the property is left unchanged.
Related examples