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plc_modules
A PLC module (an I/O card, a controller) is drawn as one master element that lists every I/O point it has, and each wire leaving it is a separate slave element that gets linked to exactly one of those points — so the drawing always shows which physical terminal a given wire actually attaches to, with its address, function text and comment carried along automatically.
This is a specific case of QET's general master/slave mechanism — the
Linking elements page (§2, §5) documents the
.elmt data format this is built on. This page is the workflow: how to
author a PLC master's I/O table, how to link a slave to a specific entry,
and what the linking widget actually shows you.
Source:
sources/ui/plclinkwidget.{cpp,h}(linking UI),sources/editor/ui/elementpropertieseditorwidget.cpp(I/O table editor),sources/editor/graphicspart/partplctable.{cpp,h}(symbol placeholder),sources/properties/elementdata.h(thePlcIOdata itself).
In the Element Editor, under File → Element properties, set the element's type to Master and its master type to PLC module. This does two things the ordinary master editor doesn't:
- Hides the max slaves and contact groups controls entirely — a PLC master doesn't cap how many slaves may attach to it as a whole (see §5 for what actually limits it).
- Replaces them with a PLC Configuration group: an I/O table you build
row by row with
+/-buttons.
Each row is one I/O point:
| Column | Holds |
|---|---|
| Type | one of six values — see below |
| Address | e.g. 1.0, 1.1
|
| Function | free text, e.g. Motor K1
|
| Annotation | free text |
| Cross-reference | auto-filled once a slave links here — leave it blank |
A second table alongside it sets how many terminals that row has (1–4) and
their labels; leaving labels blank falls back to T1, T2, … automatically.
The six I/O types are digital/analogue/universal, each input or output: Digital input, Digital output, Analogue input, Analogue output, Universal input, Universal output.
On the symbol's own drawing, add a PLC Table part (from the drawing tools, alongside line/rectangle/terminal) and position it where the live table should render. It's a placeholder only — you're marking where the table goes and sizing space for it, not drawing the table's contents; the actual rows come from the I/O table above, filled in at runtime from every slave that ends up linked to this master.
A slave that represents one wire from one PLC I/O point needs link_type
set to Slave, and in the state list — not the type list — pick
PLC slave. This is the one entry in that list that isn't really a
state: choosing it sets type = plc and state = Other behind the scenes,
and disables the ordinary type list, exactly as documented in
Linking elements §3.
Both elements need to already be placed somewhere in the project — the master on any sheet, the slave wherever its wire actually is. Then:
- Select the slave on its diagram and open its properties. Because its type is PLC, the properties panel shows an Programmable Logic Controller (PLC) tab instead of the ordinary single-link widget.
- That tab lists every PLC master in the project as an expandable tree —
Master label (folio N), each expanding to its I/O rows with columns for Label/Type/Address/Function/Annotation/Terminals. - Already-linked rows are struck through and greyed out, with a tooltip naming which slave holds them — they can't be selected.
- Right-click a free row → Log in. The link is made immediately and is undoable like any other edit.
Once linked, the tab changes shape: the tree disappears, replaced by This element is already linked, a Unlink button, and View this item (jumps to and highlights the master on its sheet). Unlink first if you need to attach the slave somewhere else — there's no direct re-link.
- The search field filters live, across every master's rows at once — by label, type, address, function or comment text, whichever column matches.
- "Hide connected elements" hides every already-linked row (and collapses a master entirely if all of its rows are taken). This is a per-user preference, remembered between sessions — check it once and it stays checked.
- The cross-reference column fills in once you link — don't type anything into it yourself while authoring the master; it's overwritten by whichever slave ends up attached.
- Terminal labels come from the I/O row's own terminal-count/labels table, not from the slave.
-
The master's drawn table repaints live when a linked slave moves —
MasterElement::connectSlavePositionUpdates()wires slave position changes straight to the master's repaint, so nothing needs a manual refresh after moving a wire.
An ordinary master (coil, protection, commutator) with max_slaves set
disappears entirely from linking UIs once it hits that count — the
"hidden" note in this widget exists for exactly that case. A PLC
master never does this, checked directly: max_slaves is forced to -1
for a PLC master (§1), and MasterElement::isFull() has no PLC-specific
branch — it only ever checks max_slaves or contact-group count, neither
of which a PLC master has.
So a PLC master whose I/O table is completely linked stays visible in
the tree — every one of its rows will simply show struck-through, with the
master item itself collapsing out of view only if the hide linked
checkbox is on and hides every child. It is not reported as full, because
per-row capacity (one slave per I/O entry) and whole-master capacity
(max_slaves) are two different, independently-tracked limits, and only
the second one is what "full" means anywhere else in QET.
Same rules as any master/slave pair: a link needs matching types (a PLC slave can only take a PLC master's I/O entry, never an ordinary master's slot, and vice versa) and a free target. Everything else is different in shape:
| Ordinary master/slave | PLC master/slave | |
|---|---|---|
| What a master offers | up to max_slaves slots, or explicit contact groups |
one row per I/O point, no overall cap |
| What "full" means | slot/group count reached | never, at the master level — only per-row |
| What the link carries | which contact this slave represents | which I/O point, with address/function/comment |
| Where you pick the target | inline on the diagram (single-link widget) | a searchable, filterable tree of every master in the project |
See also: Linking elements · Elements XML
