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I read that pins 10 and 11 are the 9V AC +/- pins on the Commodore 64 User Port. Pin 10 is the + phase while pin 11 is the - phase.
I realize the User Port wasn't as versatile as the Expansion Port (or so I imagine) but why would anyone need it anyway?
What kind or peripheral would typically require AC current?
The 9VAC available at the Commodore 64 User Port was used to drive additional peripheral loads that were not convenient to drive from the +5 VDC supply, or when doing so might exceed the limited current capacity for the DC supply. Both supplies were rated for 100 mA, so you could carry significantly more load by utilizing both supplies. Such uses also existed internal to the C64, where the system's 9 VAC was used to derive the voltages for the SID (~12V) and for the cassette motor (~ 6V).
An excellent example of this is the VIC-1011 RS-232 adapter, which uses the User Port 9 VAC in the generation of the 12V signals needed for RS-232. A schematic for the VIC-1011 shows this.
Common peripherals for the User Port, besides RS-232, were modems, printer interfaces, and EPROM programmers. A number of retro modems are designed to utilize 9 VAC, but I don't have a specific reference showing a modem for the C64 that used it. This EPROM programmer schematic does use it.
With flexibility as a goal, Commodore may have also meant for the 9 VAC supply to be useful for small motors, or to serve as a stable frequency input, as was also done with 9 VAC internally driving the TOD clock.
Possible uses would have been for any peripheral that needed:
Its own low-noise voltage regulated power supply (eg ADC/DACs for measurement and control, or audio stuff)
A symmetric power supply for analog circuitry - such can be easily achieved using a voltage doubler instead of a rectifier (caveat: you need to know what you are doing if you share a common ground with circuitry fed via a bridge rectifier!).
48V for POTS (telephony) line interfacing (could have been stepped up via a transformer)
21 to 29V (also via transformer) for EPROM/EEPROM programming equipment
This was a time before cheap and easy-to-design dc to dc converter technology became ubiquitous.
