Devices (such as modems) may NOT draw power from the Macintosh serial
ports. On the Macintosh 128K, 512K and 512K enhanced (which use DB-9
connectors for the modem and printer ports), Pin 6, the +12V pin, is
strictly a power-on indicator and may not be used as a power source of any
kind.
Macintosh Plus and later computers (which switched to Mini DIN-8
connectors for the serial ports) lack the +5 and +12 Volt signals from the
earlier models.
Despite warnings from Apple that the +5 and +12 Volt signals found on the
DB-9 serial ports on the 128K and 512K Macintoshes could be removed in the
future, some manufacturers designed peripherals that relied on one or both
of those signals. These peripherals (e.g. Thunderscan) may not work
properly on a Macintosh Plus or later Macintosh due to the removal of the
above-mentioned signals. If this is the case, contact the manufacturer of
the peripheral for a suggested solution. Some manufacturers offer
external power supplies to take the place of the absent signals.
This change also means that a Macintosh application may have to control a
peripheral's DTR line.
The serial ports from the Macintosh Plus and later models do not provide
power for any peripheral devices. The DTR line (Pin 1) is intended to
provide an output handshake signal only, NOT to power external devices.
Any devices connected to the serial ports of the Macintosh Plus or newer
models must get their power from an external source, separate from the
Macintosh system. Apple does not support devices that receive their power
from serial ports.
The solution is to use a separate power source to supply the voltage needed
for these modems or peripherals.