UART

after reading User:Pixeldoc2000's description of the SD4000#UART UART port on the SD4000, i investigated this a little and located the UART connections for all cameras i had on hand...

about the UART port in general

it appears that every canon camera has a serial port that gives access to a shell (command line interface) that lets you control the camera even without chdk loaded. it might also be useful to recover possibly bricked cameras by giving access to a bootloader.

on all cameras you get access to the Event Shell, DryOS cameras also have a DryOS Shell.

locating the uart connection points

this is relatively simple if you have access to an oscilloscope or logic analyer. it took me only a few minutes on most cameras.

the camera will always send a short string (just the event-shell prompt) when booting. so just probe on the pcb while power-cycling the camera until you find that transmission, then you have the TxD pin... RxD is usually right next to it.
(todo: note idle voltages of both pins, add picture of signal on scope).

almost all camera have an (usually unpopulated) debug/factory test connector on the pcb, usually close to the USB port. and usually the first pin on that connector is RxD and the second is TxD. also both signals are in most cases available on test-points right next to the connetor (more convenient to solder). (GND can be grabbbed at the connector shield/mount, or any convenient point.)

see the pictures below for some examples.

settings/usage

to connect to the port you will need a serial interface with 3.3V TTL levels. these can for example be bought on ebay ([search for "ttl rs232" and include international sellers], they are just a few bucks, with shiping from china).

UART Settings: 115200/8/1/N (115200 Baud)
Voltage Level: 3.3V

uart connections for some cameras

SD4000

IXUS40

Powershot_A530

IIXUS 750

PowerShot S2IS

i had no luck here, the debug connector is actually soldered in, but you need to almost completely disassemble the camera to get to it, it's a ZIF connector for a ribbon cable of very fine pitch that i did not have around. also be careful, this camera appears very sensitive due to high integration and the many PCBs... i actually broke one camera when taking it apart. :(