Raspberry Pi 4 with Mesa 7c81

maurice wrote:

From what I have seen on the fourms, this is a more unusual setup, and I will need to make and flash a custom bitfile to my board.
My understanding is that I will need PWM + 3StepGens + 3QCounts.
 

You will only need the quadrature counters if you plan to close the loop in LinuxCNC rather than in the stepper drivers. 

I guess you want the PWM for spindle control? 
 

That's right. I want to close the loop in LinuxCNC. That way it will throw an error if the actual position has deviated too much.

Has anyone experimented with closing the loop in the driver, and then also reading the same encoder's position into LinuxCNC? Or is this the usual way of doing it?

Yes, PWM Would be for spindle control.

You need a interface card that accepts RS-422 differential signals and
converts them to single ended. I would suggest a 7I85S which has 4
single ended or differential encoder inputs and 4 buffered 5V step/dir
outputs.

maurice wrote:

Has anyone experimented with closing the loop in the driver, and then also reading the same encoder's position into LinuxCNC? Or is this the usual way of doing it?

It would be simpler to just trust the driver to send an alarm signal if the position is lost.

Though, I admit, if I was doing it I would want to get the position back to LinuxCNC too.

Mesa make an encoder signal splitter, but you might not need that, especially if the drive has a mirrored encoder output (as some do)

Thank you so much.

I think I will just get stuff working by closing the loop through the driver for now. That would only require the Stepgens, which one of the included bitfiles has enough of.

If I wanted to get the encoder signal to both LinuxCNC and the driver (no mirror), what advantages would the signal splitter have over simply splicing the wires?

For the 7I85S, I assume that I would need to flash a special bitfile to my 7C81? I don't see any 7I85S bitfile in the downloads.

maurice wrote:

If I wanted to get the encoder signal to both LinuxCNC and the driver (no mirror), what advantages would the signal splitter have over simply splicing the wires?

No advantage if splicing the wires works. But it might not.

For the 7I85S, I assume that I would need to flash a special bitfile to my 7C81? I don't see any 7I85S bitfile in the downloads.

That's one for PCW but I found a VHDL file "PIN_7C80_7I85D_54" in the source. I am not sure what the "D" signifies though.

andypugh wrote:

I am not sure what the "D" signifies though.

I would venture a guess at DPLL feature ...

Here is a bit file for a 7C81 and 1 or 2 7I85S's:

 

Haven't had much time recently to work on this stuff, but thank you so much for the bitfile!

I plan to retrofit old CNC lathe and keep original servo and spindle drives and motors. All drives are +- 10V analog input and motors have TTL encoders. My idea is to use 7c81 with 7I33TA. Is this configuration workable? Do I need some special bitfile for this combination?

Thank you for help