Mesa 7i92 .bit file

I'm making a lot of progress. I got the steppers, the limit switches, and the esd working!

This CNC is for a lathe. I want to be able to thread. Can I get by with just a single index on the spindle, or would I be better off with multiple signals (which I can get off of the 10-tooth spindle lock gear)?

A single index will work, but (depending on spindle drive speed regulation),
variable threading loads and low speeds will be problematic as there's
no way for LinuxCNC to track the spindle speed variations between index pulses

If you assume relatively poor speed regulation, the threading error from speed variations goes down
approximately proportional to the square of the counts per turn so your 10 count gear might have
almost 100 times better tracking than index alone

I do have to pick up threads from time to time when I cut them on the lathe. I've had the bit break, or had to readjust the holder, had the part turn in the chuck, or even had to put the part back in the lathe to cut threads a little deeper. So this capability is something that I probably need.

My BOB has only 5 inputs. I've used three of them already for estop, x limits/home, and y limits/home. I have two left.

The G540x2D.bit setup that I used to program the FPGA has a setup for an encoder, but it uses three pins and is set up for a quadrature encoder. I looked up the hm2_7i92.0.encoder.00 commands, and see that it uses 3 or 4 pins. So, at least with the hardware I have, that's a no-go. I currently have num_encoeders=0 set in my loadrt hm2_eth statement.

Is it possible to use two GPIOs for this? Any example of the .hal configuration?

You can use a 2 pin (A+index) encoder interface with GPIO and use software encoder hal component but you are limited to
fairly low encoder resolution*speed by the fact that you can only sample at the servo thread rate

So for example if you wanted 2 1ms samples per tooth width that's 2ms per 1/20 of a turn (10 teeth 50% duty cycle)
so 40 ms per turn maximum (=1500 RPM)

Other options:
Use a custom config with A+index only hardware
add another breakout board to P1

Thanks for the input. Looks like I have several ways I can go. Although maximum speed on my lathe is 2500 rpm, I have not run it more than about 800 rpm, so I'll start with the A+index option. Higher speeds are a possibility, though. I have used another CNC lathe, and used CSS (constant surface speed). For that lathe, I was over 2000 rpm at small diameters. So if that's something I want to pursue, I'll gang the X and Z limit switches and free up a pin. Another options would be to put the ESD on either the stepper enable line or on the power line, and not go through the software at all.

And of course, I can fairly easily add another BOB with the extra port on the 7i92, if I want to add other features in the future.