A secure communications system has at least one processor and a control bus. A number of ports, each having a different fixed address, are coupled to the control bus. The processor assigns each port the address of another port whose data the port is permitted to receive when placed on a system data bus by the other port. A time slot generator outputs each fixed port address sequentially during corresponding time slots in a recurring manner, and a time slot bus is coupled to the time slot generator and to each of the ports. The ports are configured so that (a) when a given port detects its fixed address on the time slot bus, it writes desired outbound data on the data bus, and (b) when the given port detects its assigned port address on the time slot bus, the given port reads data off the data bus.