nick hull writes:

I want instant pictures. I also would like to take pictures
from an external trigger signal.

All the Canon DSLRs wake up from sleep and shoot from the shutter remote,
which is nothing but simple switch contacts. Standard 2.5mm 3-conductor
plug on the 300D/350D/400D, or the proprietary N3 connector used on the
more expensive models can be cannibalized from a $10 import remote.