At this point we know of three PET specific printer adapters, the ADA1200, the ADA1450 and the ADA1800.
The first two perform the same function, in that they are both for a serial printer. But, the ADA1450 is much more elaborate than the ADA1200, which makes me think there was something the ADA1200 couldn't do.
The ADA1200 is currently easy to get, in the UK at least, but I cannot find any 1450's or 1800's for sale anywhere.
When my ADA1200 arrives, I will be able to document it accurately, it also comes with software and a moderate amount of paperwork.
Maybe the ADA1200 was dependent more on software than hardware, but the 1450 and the 1800 did most of the work in the hardware, explaining the extra complexity and many extra IC's.
It would be good to have all these adapters fully documented and understood.
(So far we also know that the 1200 & 1450 units have a Baud rate generator based on a 555, a 10 Turn pot and a Polystyrene timing capacitor).