hw stepper card, linuxcnc, top view?!

if someone can point me to a top level explanation how these hw steppers work .. some high level block diagram or something ... I'm scrounging the Internet for hours without results .. I found some info about modbus but can't find much info about how mesa works .. the way I'd probably design it top level would be to "communicate" between linuxcnc and the external device (eth, usb, pci, pci-e device) sending "instructions" and not sending "steps". Probably "stepping" the device that's showing up as registers that translate to IO is easiest but that would be just a big IO expander, not a "hardware stepping"... are mesa cards these io expanders or actually do something in hw? I'm unable to find info .. also I'm thinking about how linuxcnc (if i understand correctly its HAL that speaks with the externals) talks to this external devices, what the basic "packet" is? is it "step, step, step..." or "take pinX, step 328 steps at 12.7456 kHz"?

I'm designing a machine where I'm needing 1 preferably 5MHz stepping (lasers, mirrors...), I can make some hw stepping done from fpga but need to send it info what to do, now if I do my own g-code player it's rather simple but I'd like to take advantage of linuxcnc being tried and tested and ease of gui adaptation .. so .. looking to buy / make an external stepper ..

mesa prices are interesting but I can't find a proper explanation how they work, which ones of them work properly with linuxcnc, what their real specifications/capabilities are ..

that's what I was looking at initially (pci-e solution) but then started to wonder how that actually work ... as if the ETH solution works the same as PCI-E solution (only bit slower) then the linuxCNC is sending "commands" to the mesa .. and from that I can conclude that linuxCNC can send that to device of my making too .. and I can probably make something more customized for my needs (probably won't as these mesa cards are so cheap but I'm still intrigued how it works) ...

Now, I have to keep asking questions as it looks like my searching capabilities go lower every year ... I'm planing to control stepers (and servos with dedicated drivers so for the linuxcnc they behave just like a steppers, as no feedback go back to linuxcnc) .. 6 of them + maybe 1 or 2 servos directly from linuxcnc .. 7I76-6I25 is the way to go? or maybe another daughter?

can I have on single 6i25 one 7i76 and one 7i77 ?

how good 7i77 are with steppers?

I'm mostly running steppers and servo's with dedicated controlers (so input for servo is enable/direction/step just as for stepper) .. but I'm thinking of maybe using 2 additional servos for the base of the machine .. I'm not using those attm, it's basically a long shot, but would be cool to know if I could do it ...

also, I'd like to add a separate high precision encoder to one of the existing motors and only monitor position by linuxcnc, the encoder would go on lathe spindle, it's 3 phase fr controlled from 0-10V input ... idea for start is to just know the position and maybe later even control that 0-10V from the mesa ...

perfect!!! thanks a bunch ...

one more question if you don't mind

I just opened this box I dedicated for that work and I see I have both free pci-e and pci slots .. the 20e difference between pcie and pci version makes no difference to me so moneywize it's irrelevant .. but any difference between 6i25 and 5i25 except for interface towards computer (pci vs pci-e)? any latency difference between pci and pci-e with linuxcnc?

perfect, many thanks for confirming ... so I can have 1 THz stepping if I want (can execute on the hw step generator) and I communicate to step generator ever "servo period".. great .. so in theory I can use usb/ethernet/uart.. too to make the hw stepper as long as I can achieve the "data exchange" every "servo period"... nice thanks!! off to read some source