maximum pulse frequency

How many pulses per turn?

This depends very much on the application you have. The details are the maximum rotating speed, the maximum number of turns you want to count, the resolution needed, the with of the index stripes and the slit width of the photointerrupter.

Maximum Pulse Frequency

When you multiply the maximum rotating speed with the number of the stripes on the disc you will get the maximum pulse frequency. E. g. 1500 rpm = 25 turns per second. The disc on the picture has 36 stripes. The pulse frequency is 25 1/s * 36 = 900Hz.

This frequency can easily be handled by your photointerrupter. When you want to reduce power consumption of the photointerrupter look at the last post to get good pulses.

That's correct as far as it goes but if you want to count a 900 Hz square
wave you need to read (sample) the detector at a minimum of 2 * 900 Hz
or 1800 Hz.

This assumes perfect square waves and perfect sampling.
Practically speaking you may need to sample at closer to 3 times
the pulse frequency. to avoid miscounts.

If you are reading quadrature ( A and B ), the minimum sample rate for
perfect signals is 4 times the per channel pulse frequency and for real
signals perhaps 6 times the per channel pulse frequency.

This assumes perfect square waves and perfect sampling.
Practically speaking you may need to sample at closer to 3 times
the pulse frequency. to avoid miscounts.

If you are reading quadrature ( A and B ), the minimum sample rate for
perfect signals is 4 times the per channel pulse frequency and for real
signals perhaps 6 times the per channel pulse frequency.

Please explain this I'm using a 20kHz breakout board
if my max rpm is 1500
60 slots encoder disc

sample rate meaning?
Can I use this bob to quadrature encoder
for real signals perhaps 6 times the per channel pulse frequency what is this meaning of this?

For LinuxCNC parallel port systems, the sample frequency is the base thread rate
(1/ base thread period)

Note that the maximum base thread rate is determined by your CPU and how it is set-up.

The highest practical base thread rate can be determined by running the latency test
on your PC, have you done this?

The breakout boards 20 KHz limit may limit your maximum encoder counting
rate but the maximum counting rate is more likely to be limited by your base thread rate.

With 60 slots and 1500 RPM you get a maximum frequency of 1500 Hz
so using my guide (3x for straight counting, 6X for quadrature) you would need
a base thread rate of 4500 Hz minimum to read the 60 slot encoder at 1500 RPM
(A only) or 9000 Hz to read the same encoder with quadrature ( A,B )

Note that these assume the slots are 50% open so you get close to square waves

I have done latency test with laptop and it is about 84000.

Note that these assume the slots are 50% open so you get close to square

One confusion I need to clear.

7i92 is used. Now, if one axis require around 80k steps per second and machine has 4 axes total, should I consider 80k multipled by 4 for the step generator??

On a 7I92, the step rate limit is per channel, the number of
channels does not limit the rate.

That is, a 7I92 can have its maximum 10 MHz step rate on
multiple axis at once.