About the UART port in general Edit
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.
NOTE: for general purpose remote camera control, CHDK's PTP Extension is likely to be more convenient, provided USB is available.
locating the uart connection points Edit
this is relatively simple if you have access to an oscilloscope or logic analyzer. 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. (but testing RxD takes a little more care, as you have to connect an external signal here, and might break something when connecting to the wrong pin.)
(todo: note idle voltages of both pins).
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.
to connect to the port you will need a serial interface with 3.3V TTL levels.
UART Settings: 115200/8/1/N (115200 Baud)
Voltage Level: 3.3V (TTL)
USB to Serial Interfaces 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).
FTDI sells USB TTL Serial Cables like TTL-232R-3V3 which are perfect for the job.
Bus Pirate is also a very nice hacking tool (supports UART 3.3V & 5.0V, JTAG, I2C, SPI and more).
Changing UART settingsEdit
On VxWorks cameras, it is possible to change the UART speed etc. using VxWorks ioctl calls. This can be done from CHDK Lua using the Lua/Lua Reference/Native Function Calls call_func_ptr function. To do this, you will need the addresses of the following functions: open ioctl close
These can usually be found in the stubs_entry.S file for your platform in the CHDK source tree. The following Lua example changes the UART speed to 9600 for a530 (note, based on user report, actual code is untested! Speed down to 300baud tested and works on a530)
FIOBAUDRATE=4 -- vxworks IOCTL number, found on google -- function addresses from stubs_entry.S fptr_open=0xffec8b34 fptr_ioctl=0xffec8630 fptr_close=0xffec84f0 -- open the file console device r/w fd=call_func_ptr(fptr_open,"/tyCo/0",2,0) print('open returned',fd) -- >= 0 indicates open works if fd < 0 then error('open failed') end -- call the ioctl status=call_func_ptr(fptr_ioctl,fd,FIOBAUDRATE,9600) print('ioctl returned',status) -- 0 probably means success -- close the fd call_func_ptr(fptr_close,fd)
uart connections for some cameras Edit
in all my pictures:
red=TxD (this is an output, be careful not to short it, that might break the camera)
SD900 also have UART. ToDo: add PCB Picture and Pinout.
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. :(