Raspberry Pi 4 with Mesa 7c81

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08 Oct 2021 22:33 #222596 by andypugh

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? 
 

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08 Oct 2021 22:46 - 08 Oct 2021 22:49 #222602 by maurice
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.
Last edit: 08 Oct 2021 22:49 by maurice.

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08 Oct 2021 22:49 - 08 Oct 2021 22:51 #222603 by PCW
Replied by PCW on topic Raspberry Pi 4 with Mesa 7c81
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.
Last edit: 08 Oct 2021 22:51 by PCW.

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08 Oct 2021 23:00 #222606 by andypugh

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)

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09 Oct 2021 18:24 #222673 by maurice
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.

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11 Oct 2021 11:10 #222842 by andypugh

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.

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11 Oct 2021 11:39 #222846 by tommylight

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


I would venture a guess at DPLL feature ...

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11 Oct 2021 15:07 #222857 by PCW
Replied by PCW on topic Raspberry Pi 4 with Mesa 7c81
Here is a bit file for a 7C81 and 1 or 2 7I85S's:

 

File Attachment:

File Name: 7c81_7i85sx2d.zip
File Size:114 KB
Attachments:

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16 Oct 2021 22:52 #223333 by maurice
Haven't had much time recently to work on this stuff, but thank you so much for the bitfile!

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