the virtues of locking

I implemented Tim's suggestion to do Babbage-style consecutive locking from source to destination, instead of engaging all the locks at the same time.  Each locking takes half a time unit, so doing all three locks takes1.5 time units. That conforms to what Babbage seems to have indicated in his timing diagrams.

Nothing changed in the operation, though, so it's not clear under what circumstances it makes a difference. Maybe it allows more misalignment to be corrected.

My new little tester continues to copy numbers with 100% reliability. I decided to do an experiment to see if the new lock on the fixed long pinion had anything to do with it, so I pulled the plug to its stepper motor.

The machine failed immediately.

The problem was a jam when the small pinions tried to mesh with the digit wheels. I think the issue is not that the new lock corrects small misalignments after the operation, but that it prevents derangement when the four-gear pinion train (small to fixed long to movable long to small) is not otherwise meshed to anything that prevents it from moving.

Maybe the rule shouldn't (only) be "lock every third gear in a train", but "make sure every non-driven train is locked somewhere".