What is the maximum voltage that the modem or printer port can handle on the "receiving data" pin of the Macintosh serial port.
Engineering (and the manuals describing the receivers) indicates that the serial ports should be able to handle voltage ranges of +/- 25V.
The Quad Differential Line Receiver used on the Power Macintosh 6100, 7100, 8100, 9150 and the Quadra 630 family is rated to a maximum of +/- 25V on any differential input.