Altium Designer Documentation

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

Created: July 27, 2015 | Updated: September 26, 2016
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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 auto-junctioning capability allows you to create electrical connections at junction points without needing to manually define the connections through the placement of manual junctions.


This type of junction is placed automatically by the Schematic Editor's 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 2 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 autojunction 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.

Defining a Junction at a Cross Over

When one wire/bus/signal harness crosses directly over another, no connection is created (a compiler generated junction is not automatically placed). If a connection is required in this situation, a Manual Junction must be placed.

If a cross-over style junction is required, a
manual junction must be placed.


A Compiler generated junction cannot be edited in the usual manner (via a dialog 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 juntion instances will be, hotspots are used. Enable the use of these hotspots, and specify their color - for wires and buses - also as part of your preferences.

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 to 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, a couple of 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 to the cursor as you drag.

This feature requires the Display When Dragging option to be enabled in the Auto-Junctions region, on the Schematic - Compiler page of the Preferences dialog.
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 occuring in an area of the circuit beyond the currently viewable area of the workspace.

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


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