The PowerBook 100 had one external serial port, designed for use with a printer or any compatible RS-422 device. It was the first Macintosh to omit an external modem port, instead offering an optional built-in 2400 baud modem for communications. As a result, for the first time a user could not print directly and access AppleTalk or a faster external modem simultaneously, and devices such as advanced MIDI interfaces could not be used because they required the dedicated use of both ports. A third-party serial modem port could, however, be installed in the internal modem slot for consumers who needed traditional functions.