Techno Stuff
How They Work - Index Units

A quick look at a couple online dictionaries shows that this definition of the word index is not exactly what you'd call common usage, but the enginerds out there have probably run across the term.

For our purposes, indexing means to stop a cam, ratchet, or a positioning cam from rotating. A positioning cam is usually a metal disc with notches. The notches are used to stop a mechanism in the correct alignment. For example, the magic screen shouldn't stop moving until the panels are lined up over the numbers correctly, or a magic square needs to always rotate in 90 degree turns.

Let's take a peek at the usual timing/switch lifting cams.

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index unit in indexed position

Here we are looking from the motor end of a control unit shaft. We see the notch in the timer cams assembly, and the pin on the index arm is engaged in the slot, so the cams can't turn. Note that many index units also have switch stacks on top of them. In this case, the top switch in the stack looks like it's incorrectly adjusted ... it appears to be open.

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index unit in released position

When it's time to release the cams, the coil pulls in and the index pin is lifted out of the notch. As long as the coil has power, the pin is held away from the cam. The switches on the unit have changed state. Sorry, I forgot to rotate the image before adding the text. Too lazy to do it again.

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index unit in returning position

Here is the tricky case. When power is removed from the coil, the index arm spring pulls the pin against the edge of the cam. When the notch rotates under the pin, the pin falls in and stops the cam from turning.

The reason that this is a tricky case is because of the switches on the stack. Are the switches supposed to be open or closed when the coil power is removed? The answer depends on which index unit you are talking about.

The game manuals usually have a note in them that the replay cams switches need to change state only when the coil is powered (the switches in the indexed and returning positions must be the same). Screw this up, and the obvious symptom is payouts that round up to some multiple of usually 12 - e.g. a 8 win pays 12. A less apparent problem is that the replay counter disc printed circuit traces can get burned through over time.

The timer cams switches, on the other hand, are in one state in the indexed positiion, and in the other state for both the released and returning positions.

You can usually just read the description of the switch function in the manual and it's pretty obvious how the switch needs to work. For example, one of the timer cams switches usually keeps the power on the motors. It makes no sense for the motors to turn off when the cams are in the middle of rotating, so that switch must be closed unless the cam is indexed (it's a carry-over switch).