A pool or spa control system includes, a main control panel housing a motherboard, relay bank, and local terminal. The motherboard includes a processor in two-way communication with a relay bank socket via an internal bus. The relay bank is connectable to the relay bank socket and includes a processor, memory, plurality of relays, connector, and an internal bus establishing two-way communication between the relay bank processor and the motherboard processor when the relay bank is connected to the relay bank socket. The local terminal includes a control processor, user interface, and memory, and is in two-way communication with the motherboard processor for allowing user control of the system. The control processor automatically discovers and assigns the relay bank a network address upon connection of the relay bank to the motherboard relay bank socket, and the relay bank returns relay bank parameter information, which the local terminal stores in memory.