M-32TL Timing: Why three position sensors?

While reviewing my last post I realized that I'd left out an interesting behavior that happens after the printer hammer fires.

Let's look at a couple of waveforms captured using an intact P170-DH calculator. All of these are taken from the same capture, covering the printing of the first 6 characters on a line. The sensor signals are labeled based on the operation I expected when I started working with this printer rather than the nomenclature I use now.  In each case I've placed cursors at the leading (falling) edges of the "Rotation Index" signal; these mark the start and end of one complete revolution of the print wheel.

As shown by the transition of the "Motor Run" signal, this is at the very beginning of a print cycle when the motor is just starting. The calculator logic waits for one full rotation of the print wheel before starting to print, probably to allow it to come up to speed and make sure the logic is synchronized to its rotation.

Since nothing is going to be printed, the "Hammer Fire" signal remains inactive throughout this period. My logic analyzer reports 28 transitions of both of the two sector sensors, 14 falling edges and 14 rising edges each. There's nothing unexpected here.

Now let's move to a period where the first character is being printed. I've also moved the two cursors so they are again aligned with the leading edges of the "Rotation Index" symbol.

Again, the signal labeled as "Start of Symbol" shows 28 transitions in one complete revolution of the wheel. This is necessary to keep the logic in sync with the wheel position. The long gap between these pulses indicates pretty clearly that the print wheel, and the sensor disc attached to it, stops moving for 35 to 40 milliseconds when the hammer drives the wheel against the paper.

What's more interesting, though, is the signal labeled as "End of Symbol". During this print cycle it has 30 transitions rather than 28. Since all three signals are derived from contacts on the same sensor disc, this suggests that the disc actually rotates backward, resulting in multiple transitions of this signal.

This doesn't always happen just once. Here's a later period where 32 transitions of the "End of Symbol" signal were recorded.

Here's the same print cycle zoomed in to show the chatter more clearly. At a sampling rate of 20 KHz I only captured a sample every 50 microseconds so there could have been more.

My guess is that printing occurs with this sensor just on the edge of contact, so any small wiggles in the sensor disc can cause multiple closures. This would make it impractical to use only two position sensors in this printer.