Fanuc retrofit retaining original AC servo motors and drives

I don't think it has actually been done yet. So you have a choice between an interesting project or replacing the drives.

It is probably worth trying the "project" first, I think initially that just requires a suitable firmware and some TTL to (whatever) conversion.

It looks like it will be a lot easier to swap to analog drives. The digital drives would require 6 pwm and 7 encoder signals per axis.

Th next question I have is about layout. On my lathe I stuck the motherboard right behind the touch screen as this was also close to the servo amplifiers. On the mill, from the control panel to the main electrical box would take at least 15 ft of wiring. Do I put the motherboard with 5i24 in the main electrical box and run long monitor/ keyboard cables, and also put a remote I/O board in the control panel? Is there a speed advantage to use a ribbon cable connected 7i48 servo card vs a serial connected 7i83 remote servo card?

I suggested the 7i84 as a card with lots of pins for PWM. If you are not gonna that way then maybe an Ethernet card is the way to go? It would simplify wiring.

I purchased the components for the computer, and now I need to order Mesa boards. I counted that I will need approximately 100 gpio signals on top of the encoders and servo signals.
Here is what I am thinking:
6i24 PCIE FPGA card
7i48 6 channel analog servo
7i46 SPI breakout board
4 or 5 7i50 I/O expansion cards

Does this sound like a reasonable combination?
Can the 7i50 handle a MPG encoder?

What voltages are your GPIO signals? Most of the Fanuc machine I've worked with are running 24v io. A 7i50 is only 5v. I think some of Mesa's Smart Serial daughter cards may be a better option.

To interface to the the Digital Fanuc motor drives requires

1. 2 analog inputs per channel (2 phase current feedback channels)
2. 8+ TTL level or differential signals (depending on drive model)
6(TTL) or 12 (Differential) of these signals are the three phase PWM
which must reverse direction based on the driver ready signal,
as they are used for fault signaling when the drive is not ready
3. The host (linuxCNC) or a smart controller must implement commutation and FOC

This may be something better done with a STMBL drive with no power circuit
or a custom FPGA card with the correct I/O built on board