Serial floppy drives

There's a lot more difference than that, and the voltage levels quoted are wrong, or at the very least, misleading. I don't have a copy of the relevant standards handy, so I might have some details wrong, but here goes... RS-232, which is now called EIA-232, is a single-ended signalling scheme. At the transmitter, the voltages are nominally around +12V (mark) and -12V (space), but are actually allowed to range from +25V to -25V. Aside transmitter including short-circuit protection. At the receiver end, voltages above +3V are considered a mark, and below -3V are considered a space. A signal between +3V and -3V does not meet the EIA-232 specs, so the behavior is undefined, but most receivers will consider a signal near 0V to be a space. The documentation is wrong. EIA-422 is a differential signalling standard. Rather than having a data signal referenced to ground, you have two signals referenced to each other, e.g., RxP and RxN for receive, and TxP and TxN for transmit. EIA-422 cannot interoperate with EIA-232 without a converter of some kind. However, the Macintosh ports also support EIA-423, which is a single-ended signalling method using levels approximately equivalent to those of EIA-422. In many cases EIA-232 equipment and EIA-423 equipment can interoperate (as is usually the case with Macintosh serial ports and EIA-232 devices), but technically an EIA-423 device does not in fact meet EIA-232 specifications. Another difference between EIA-232 and EIA-422 is that EIA-232 specifies a connector and the assignment of functions to connector pins, while EIA-422 and EIA-423 specify electrical characteristics only. Note that a device that uses an EIA-232 electrical interface but not the DB-25 connector is not actually compliant with the EIA-232 specification. For instance, devices with DE-9, modular, or MMJ connectors.