Relay boards are connected to a ten-way plug on the Challenger, Access Controllers, or DGP's to expand relay capacity. The TS0840 provides an additional 3 dry contact relays to the Challenger Panel & 4 relays to the Access Controllers, or a possible 8 on a TS0820 DGP. When fitted to a Challenger Panel or Access Controller, the TS0840 cannot be combined with other relay boards.