The motor and solenoid drive circuits are garden variety Darlington drivers with free-wheeling diodes, which are no surprise. But the connections to the position sensor are marked T, R, and t, the bar over the name indicating that these are active-low signals. I was hoping this would provide some definitive answers on how this printer is intended to be used, but this is all there is on the subject.
I'm betting that R is the rotational index that identifies the first sector. That leaves T and t. The naming suggested that these indicate the start and end of the character sector, and at first I thought, "Oh, wow, Arne Rossius was right after all!" However, the timing I observe in the unmodified Canon P170-DH calculator doesn't support that theory.
Here's a screen capture from my logic analyzer showing the period when the solenoid fires:
The top trace is what I call the "character index", which I believe is used to track which sector of the print wheel is coming. The middle trace is what I call the "solenoid index", and the bottom trace is the solenoid driver itself.
There is a really long gap (about 1.7 milliseconds) between the leading edge of the character index signal and the solenoid activation. That's about half a sector away from the activation of the solenoid, so it seems unlikely that this is the trigger for the solenoid. However, there is only a very small delay after the leading edge of the solenoid index and the activation of the solenoid, and about the same delay after the next solenoid index and the release of the solenoid.
So I stand by my original interpretation.
While I'm on the subject, I came across the blog of Mike Clarke, who tore apart an M-32TL printer and took some really nice pictures of the mechanism. I wanted my printer to work after I was done with it, so I didn't disassemble mine as Mike did.
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