To clarify slightly, applying +5V to the Powershot N's USB power pins is not enough to cause the camera to power up.
However, plugging its USB cable it into a USB "master" like a PC does wake it up. As the camera's battery is charged via the USB port, that suggests the application of 5V causes some kind of "wake-up" and the Canon firmware then looks for a host on the data lines. If it does not see anything there, it powers down.
OK, this sounds a lot like the problem I had when I bought a cheap 3m long
Apple dock connector--USB cable on eBay. It worked fine with my iPhones but the iPad gave a "Not Charging" message. I did quite a bit of online research and found out that iDevices in general have a USB battery charger detector chip, which complies with a USB battery charging standard, that performs what is referred to as "port detection" and "enumeration". It detects the current a USB charger is capable of supplying by polling the voltages on the two data lines. These values are apparently custom hard-coded into the detector chip for a given manufacturer's device and must be present on the data pins of the charging device if the full current is to be utilized.
For iDevices, it turns out that the following voltages on the D+/D- data lines correspond to the given current capacity of the charger:
- Default
- 2.0/2.0 - 0.5 A
- ?.?/?.? - 0.5 A
- iPhone 5W charger
- iPad 3 10W charger
- iPad 4 12W charger
My measured voltages of the D+/D- pins of all these chargers were within millivolts of the specs. My measured voltages of the USB port on my monitor were 0.0/0.0.
If none of these voltage combinations is detected, charging proceeds at the default current of .5 A, corresponding to the capacity of a standard USB port on a computer even if there is the "Not Charging" error. Otherwise the full current, determined by the voltage combination, is deployed. The problem with the long cable (I am pretty sure) was that it had thin enough stranded wire that the resistance was high enough to drop the voltages on the data lines to the point where the detector chip would only allow charging at the default 0.5 A current and not the 2.1 - 2.4 A the iPads can handle. I'm not sure about the iPhone not reporting an error but maybe it's specs are more lenient.
From what you said, your camera may very well have one of these chips and so is looking for certain voltage combinations on the data lines. If you use a triggering device that only supplies +5V and GND, the data lines are floating. The USB port of your computer, on the other hand, is supplying some voltage values on the data lines and those voltages are apparently the ones needed for remote power on. Find out what those voltages are and you can then modify your device to supply the same voltages. It may be that they are 0.0/0.0 like mine are. Then all you'd have to do is tie both pins to GND.
You can read more about the detector chip here:
MAX14578AE
USB Battery Charger DetectorsThe Basics of USB Battery Charging: A Survival Guide
