[Ether]

The "100" in the part number EP4-100-079-D-H-T-B means 100 cycles per rev. In this context, a "cycle" means:

- a rising edge on Channel A, followed by
- a rising edge on Channel B, followed by
- a falling edge on Channel A, followed by
- a falling edge on Channel B

If only one channel is being displayed on the scope, there will be 100 rising and falling edges per rev, which is 100 cycles per rev.

So the math looks like this:

20 rev/min * (1/60) min/sec * 100 cycles/rev = 33 Hz

... which is what the OP calculated in post #1.