Page "RS-232" Paragraph 35

Because both ends of the RS-232 circuit depend on the ground pin being zero volts, problems will occur when connecting machinery and computers where the voltage between the ground pin on one end, and the ground pin on the other is not zero.

This may also cause a hazardous ground loop.

Use of a common ground limits RS-232 to applications with relatively short cables.

If the two devices are far enough apart or on separate power systems, the local ground connections at either end of the cable will have differing voltages ; this difference will reduce the noise margin of the signals.

Balanced, differential, serial connections such as USB, RS-422 and RS-485 can tolerate larger ground voltage differences because of the differential signaling.