 Since the J links were externally connected, switchroom staff discovered that such failures could be found by making busy both grids, grounding the controller's test leads, and then testing all 128 J links, 256 wires, for a ground.Supervision and trunk signalling were the responsibility of trunk circuits.The most common kinds (reverse battery one-way trunks) were in plug-in trunk packs, two trunks per pack, 128 packs per Trunk Frame (originally) on 16 shelves.