A datum dimension is a group design object. It allows for the dimensioning of a linear distance of a collection of objects, relative to a single reference object. The first object chosen is the 'base'. All subsequent objects are relative to this first object. The dimension value in each case is therefore the distance between each reference object and the 'base', measured in the default units. The references may be tracks, arcs, pads, vias, text, fills, polygons or components.
Datum dimension objects are available for placement in the PCB Editor only. Use one of the following methods to access a placement command:
Choose Place»Dimension»Datum from the main menus.
Click the button on the Place Dimension drop-down () of the Utilities toolbar.
After launching the command, the cursor will change to a cross-hair and you will enter dimension placement mode. Placement is made by performing the following sequence of actions:
Position the cursor and click or press Enter to anchor the dimension start point (this is the first reference object or 'base').
Move the cursor to the next required object and click or press Enter to anchor the dimension end point (this is the second reference object).
Move the cursor to subsequent reference objects and click or press Enter. When all desired objects have been selected, right-click or press Esc.
The text can now be initially positioned. Click or press Enter when the text is in the desired position to complete placement and exit placement mode.
When dimensioning an object, anchor points become available to you, highlighting where the dimension can be attached. The point nearest the cursor will be the one used, and where the dimension will attach if you proceed to click or press Enter.
Additional actions that can be performed during placement are:
Press the + and - keys (on the numeric keypad) to cycle forward and backward through all visible layers in the design respectively – to change placement layer quickly.
Press Spacebar to rotate the dimension anti-clockwise or Shift+Spacebar for clockwise rotation. Rotation is in accordance with the value for the Rotation Step, defined on the PCB Editor – General page of the Preferences dialog.
Press the Tab key to access an associated propertiesdialog, from where properties for the dimension can be changed on-the-fly.
While attributes can be modified during placement (Tab to bring up associated properties dialog), bear in mind that these will become the default settings for further placement unless the Permanent option on the PCB Editor – Defaults page of the Preferences dialog is enabled. When this option is enabled, changes made will affect only the object being placed and subsequent objects placed during the same placement session.
This method of editing allows you to select a placed datum dimension object directly in the workspace and change properties such as the position of its text and its reference points, graphically.
When a datum dimension object is selected, the following editing handles are available:
A selected Datum Dimension.
Click & drag any of the handles at the text end of the extensions to adjust the dimension text position for all cases simultaneously.
Click & drag A to move the base point of the dimension.
Click & drag subsequent handles to move each reference individually, with respect to the base.
All handles nearest to the object(s) being dimensioned allow for redefinable references – once the dimension is detached from a reference object it becomes non-referenced and can be moved for attachment to a different reference point or object. As you drag any of the editing handles, the dimension may be rotated.
If the datum dimension object is totally non-referenced (i.e. it is not attached to any reference design objects) click anywhere on it – away from editing handles – and drag to reposition it. While dragging, the datum dimension can be rotated (Spacebar/Shift+Spacebar) or mirrored (X or Y keys to mirror along the X-axis or Y-axis respectively).
If attempting to graphically modify an object that has its Locked property enabled, a dialog will appear asking for confirmation to proceed with the edit. If the Protect Locked Objects option is enabled on the PCB Editor – General page of the Preferences dialog, and the Locked option for that design object is enabled as well, then that object cannot be selected or graphically edited. Double click on the locked object directly and disable the Locked property or disable the Protect Locked Objects option, to graphically edit the object.
The following methods of non-graphical editing are available:
This method of editing uses the following dialog to modify the properties of a datum dimension object.
The Datum Dimension dialog.
The Datum Dimension dialog can be accessed prior to entering placement mode, from the PCB Editor – Defaults page of the Preferences dialog. This allows the default properties for the datum dimension object to be changed, which will be applied when placing subsequent datum dimensions.
During placement, the dialog can be accessed by pressing the Tab key.
After placement, the dialog can be accessed in one of the following ways:
Double-clicking on a placed datum dimension object.
Placing the cursor over a datum dimension object, right-clicking and choosing Properties from the context menu.
Using the Edit»Change command and clicking once over a placed datum dimension object.
Quickly change the units of measurement currently used in the dialog between metric (mm) and imperial (mil) using the Ctrl+Q shortcut. This affects the dialog only and does not change the actual measurement unit employed for the board, as determined by the Measurement Unit setting in the Board Options dialog (Design » Board Options).
The PCB Inspector panel enables the designer to interrogate and edit the properties of one or more design objects in the active document. Used in conjunction with appropriate filtering - by using the PCB Filter panel, or the Find Similar Objects dialog - the panel can be used to make changes to multiple objects of the same kind, from one convenient location.
The PCB List panel allows the designer to display design objects from one or more documents in tabular format, enabling quick inspection and modification of object attributes. Used in conjunction with appropriate filtering - by using the PCB Filter panel, or the Find Similar Objects dialog - it enables the display of just those objects falling under the scope of the active filter – allowing the designer to target and edit multiple design objects with greater accuracy and efficiency.
A datum dimension object can be moved in the following ways:
Selecting both the dimension object and the objects that are being dimensioned. The whole can be dragged to a new location as required.
Selecting an object that is being dimensioned only. The dimension text will follow the object in its alignment plane only. The dimension extensions will expand/contract to keep the relationship between dimension and object being dimensioned.
Selecting the dimension object only. It is important to note that the dimension cannot be moved on its own if it is referenced by a design object. To move the dimension only, it must first be detached from the objects it is dimensioning.
The dimension's value automatically updates as its start or end points are moved. Likewise, if the position of an object that a reference point of the dimension is anchored to is changed, the dimension will update and expand/contract to reflect this.
When the reference or references to which a dimension object is attached are deleted, a dialog will appear, asking whether the dimension should also be deleted. If the dimension is not deleted, it remains in the workspace, but non-referenced.
Datum dimensions are group objects consisting of text and track segments. They can be converted to their set of primitive objects by choosing Tools»Convert»Explode Dimension to Free Primitives from the main menus. Once exploded, a dimension object can no longer be manipulated as a group object.