 The FCC ruling also made it difficult to have any sizable holes in the case, which eliminated expansion slots or cards that communicated with the outside world via their own connectors.Instead, Atari designed the Serial Input/Output (SIO) computer bus, a daisy-chainable system that allowed multiple, auto-configuring devices to connect to the computer through a single shielded connector.The internal slots were reserved for ROM and RAM modules; they did not have the control lines necessary for a fully functional expansion card, nor room to route a cable outside the case to communicate with external devices.