Altium Designer Documentation

Working with a Compiler Generated Junction Object on a Schematic Sheet in Altium Designer

Created: December 14, 2017 | Updated: December 17, 2018
All Contents

Parent page: Schematic Objects

The schematic compiler automatically adds junctions at each T-junction to
complete the electrical connection.


A junction is an electrical design primitive. It is a small circular object used to join intersecting wires (or buses or signal harnesses) on a schematic sheet. A compiler generated junction is a junction that is automatically placed by the auto-junctioning feature when two wires/buses/signal harnesses are connected in a T-type fashion, or when a wire/bus/signal harness connects orthogonally to a pin, power port or other electrical object.


This type of junction is placed automatically by the auto-junctioning feature. As such, it is not a design object that can be accessed and placed by the user.


Compiler generated junctions are placed automatically whenever a T-junction occurs during wiring, such as two wires/buses/signal harnesses meeting in a T, or a wire/bus/signal harness orthogonally crossing the end of a component pin or another electrical object, such as a Power Port.

If the Break Wires At Autojunctions option is enabled on the Schematic - General page of the Preferences dialog, an existing wire/bus/signal harness segment will be broken into two at the point where an auto-junction is inserted. For example, when making a T-Junction, the perpendicular wire/bus/signal harness segment will be broken into two segments, one each side of the junction. With this option disabled, the wire/bus/signal harness segment will remain unbroken at the junction.


A compiler generated junction cannot be edited in the usual manner (i.e. in the Properties panel or graphically on the schematic sheet). The display properties of compiler generated junctions are configured on the Schematic - Compiler page of the Preferences dialog, as shown in the image below. Note that disabling the display of compiler generated junctions does not break the electrical connection at that junction point.

Configure the display options for compiler generated junctions (auto-junctions) in the Preferences dialog.

Indication of New Auto-Junction Creation

Depending on the affected wiring, performing a drag operation may result in the creation of auto-junctions at new locations. To provide visual feedback on where these new junction instances will be, hotspots are used. Enable the use of these hotspots and specify their color for wires and buses also on the Schematic - Compiler page of the Preferences dialog.

Control the display of predicted auto-junctioning during drag operations.

Example showing predicted new auto-junctions resulting from a drag operation.

Visual Indication of Connectivity Change

While dragging a component, it is possible to inadvertently drag a little too far or off-course, resulting in an unintended auto-junction and a potentially fatal change to the connectivity of a circuit. To provide a timely and graphical indication of the status of connectivity while performing a drag, the following icons are used:

  - OK - the drag operation is not altering the connectivity of the circuit.

- Alert - the drag operation is causing a change to the connectivity of the circuit.

The applicable icon is displayed near the cursor as you drag. The benefit of having the alert symbol displayed near the cursor rather than at the point of connectivity change is that you get a visual warning for a change that may well be occurring in an area of the circuit beyond the currently viewable area of the workspace.

This feature requires that the Display When Dragging option is enabled in the Auto-Junctions region on the Schematic - Compiler page of the Preferences dialog..

Providing a visual warning that a drag operation will result in a change to connectivity.

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