Allegro doesn't support tandem (broadside) pairs however they can be routed by first defining and providing a primary gap for the pair. Preroute the two clines so they are directly over each other. Originally posted in cdnusers.org by Trykon
As an example, Diff Pair DPA, comprised of nets A+ and A-, is to be routed with the following parameters as a tandem pair (each net on different layers)
* TOP layer line width = 10.0
* L3 layer line width = 10.0
* Setup the NET_PHYSICAL_TYPE and it's assignment table to route the pairs with the desired width and layers.
* Do not have a primary gap width set or do not have the pairs assigned to an ECSet
* Fanout or start the routing for each net in the differential pair and leave the end points for each net at the same XY location.
* Set a primary gap width in either the Constraint Manager worksheet or the ECSet. Ensure that the differential pair is assigned to an ECSet. This is not used to compute a true gap. It's only defined so that Allegro will use the 15.0 Differential Pair routing methodology. Of course there would be "uncoupling DRCs" if they were constrained in the spreadsheet or the ECSet.
* Use "Add Connect " to route the pair. The Active layer will control which cline is the primary of the pair.
Alternative if the pairs are already assigned to an ECSet
* Setup the NET_PHYSICAL_TYPE and it's assignment table
* Using 'Add Connect start routing by selecting one of the pins. Use RMB 'Single Trace Mode'. Digitize an XY location. Select RMB 'Done'.
* Select the other half of the differential pair. Digitize an XY location, preferably at the same location as the other half of the differential pair. Select RMB 'Done'.
* Select one of the differential pair nets. Select RMB and disable 'Single Trace Mode'. This will gather the two clines and allow companion routing. If one half of the pair is to be routed with a different width you will need to use 'Edit >Change' to set the correct line width.
Hope this helps.