Getting started without knowing where to start.

Hey there

I'm currently in the process of gathering all the bits and pieces for my first CNC Router. I have a bit of experience in 3d printers having built two voron printers ( vorondesign.com/ ) running klipper. So the concepts of driving steppers and reading/reacting sensors are well known to me.

I looked into interfacing options in the linuxcnc world and I'm a bit puzzled. While I understand the offloading to the Mesa Cards FPGA I dont really understand why most cards are built to be extended. Since I want to start simple with a Raspi 4 I would like to start with the 7c81 but it appears to be mostly made to extend and or attach other boards. Coming from 3d printing where even drivers are onboard this is seems a bit foreign. Now i know subtractive manufacturing needs more omph thus drivers are built for higher amps and thus need better cooling. I understand relay boards. But what about encoder inputs and the likes, why are these often only read on BOBs?

I'm pretty sure I'm missing something, I'm in a bit of a chicken and egg situation without a hardware to learn it.

Thanks a lot, 
Alessandro

I guess one difference between general CNC machines and 3D printers is
the 3D printers are much more consistent hardware wise and tend to use 5V signaling
rather than 24V, so it's easier to make a "one size fits all" interface card. The use of
expansion cards allows more flexibility.
(not every design will use encoder feedback or the same amount of 24V I/O)

That said, Mesa does have cards that don't need expansion other than motor
drives (a 7I96 for example)

Welcome,
not sure where you got the BOB use for encoders but that is possible at very low speeds as the parallel port is limited at pulse counts/generation.
And i did finish a Voron Legacy lately.
Regarding Mesa boards, there are other boards that work with LinuxCNC, like Pico systems and some others, but i stuck with Mesa as they are extremely versatile and they do have the best support on planet Earth, most of them can have custom firmware courtesy of PCW (the owner of Mesa) as he is always around here to help.
As for the difference between 3D printer boards and Mesa/Pico,the latter are made for machines ranging from puny hobby machines to some humongous 16 tons hunks of steel, so the drives for some of those machines come with forklifts to carry them.

Thanks guys for your responses and the contexts!

Whenever I approach a new topic I try to browse forums and youtube a bit to understand what others are using. And I came across so many chinese BOBs that it made me think it's sort of a defacto entry level standard. (Chinese BOBs are even mentioned in the 7c81 doc. )

Is there a particular reason for 24V IO for signalling? Like overcoming noise a bit better?
I think my problem understanding also stems a bit from looking at the doc of the the particular board (7c81) where I would've have expected its capabilities of IO Ports to be shown (like e.g. for this 3d printerboard schema ae01.alicdn.com/kf/H1f989fbed64049918a5503ff286b1b150.jpg ) is that because you can do everything with every port (Analog, PWM, i2c..) or because it is expected for this board to be doing this on breakout boards anyways?

So If I might ask the newbie way: What setup would you suggest for a 3 axis 4 stepper configuration for a router driven by a raspberry?

Thanks again!

Is there a particular reason for 24V IO for signalling? Like overcoming noise a bit better?

Yes, 24V has better noise immunity, is more reliable for mechanical contacts,
and is industry standard for things like proximity switches, relays, solenoids etc.

And yes, on the FPGA cards any pin can be any digital function (step/dir, PWM, encoder,
UART etc, etc)