Quote Originally Posted by MrBaz View Post

Simple TTL will still have the camera send an 'off' signal to the flash when proper exposure is met. Doesn't seem that complicated.
Film era TTL reads the light reflecting off the film itself in most systems.
Digital sensor reflects completely different than film so a different way of telling when to quench the flash is used.

Quantum modules for individual sytem ttl could be "modified" for digi when digital became available to the masses but I've read some customers less than happy with these modifications and it basically is just a potentiometer for fine adjustment.