How do I add additional serial port?

The basic idea behind GeoPort was to allow one to build a low-cost analog communications device by using the hardware already built into the Mac for most of the data handling. Early machines were not fast enough to emulate these devices, but the AV-series added an AT&T 3210 DSP which could handle this with ease, and the later Power Macintosh's PowerPC had enough raw processing power to do this without the DSP helping. Since this hardware was already built into the Mac, all that was needed externally was a line adaptor which included a digital-to-analog converter to convert the digital sound values generated by the computer to analog signals, and vice versa. Better yet, the same system could be used to handle voice, which required too much bandwidth to easily send over existing serial ports.