Mesa 7i77 support for absolute linear encoder

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28 Nov 2023 13:24 #286700 by pmchetan
I am thinking of close loop servo controlling my axis BLDC motors using renishaw SAS10-LA11 magnetic encoders. I bought 6i25 with 7i76 and 7i77 in particular to use 7i77 to generate motor control signals after getting position information from encoder inputs. Are mesa 7i77 capable of using absolute linear encoders or are they limited to quadrature, index incremental encoders?
Is there a way to program the encoder inputs to take linear encoder signals:  the absolute encoders from renishaw has BISS-C or SSI protocol encoders with both variant containing 4 pins: clock+ clock-, data+ data- and VCC and ground to power the encoder.

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28 Nov 2023 23:53 - 28 Nov 2023 23:55 #286740 by Mecanix
Good question. Much like yourself I'm on the lookout for a feedback implementation on a soon-to-retrofit mill. Lots of sine signal outputs/hardware available, matter of interfacing this with Lcnc that I'm sure you're confused about, as I am. Subscribed! ps. I'll update if I find anything relevant. 

Doc's INI configuration mentions this:

"HOME_ABSOLUTE_ENCODER = 0 | 1 | 2 - Used to indicate the joint uses an absolute encoder. At a request for homing, the current joint value is set to the HOME_OFFSET value. If the HOME_ABSOLUTE_ENCODER setting is 1, the machine makes the usual final move to the HOME value. If the HOME_ABSOLUTE_ENCODER setting is 2, no final move is made."
Last edit: 28 Nov 2023 23:55 by Mecanix.

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29 Nov 2023 12:58 #286777 by PCW
For a BISS or SSI encoder you need RS-422 interface hardware
A 7I74 or 7I85 can provide this interface. The 7I77 has only one
RS-422 interface (the serial expansion port)
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30 Nov 2023 07:18 #286848 by pmchetan
Three questions:
Can the RS422 on 7i77 be used as encoder input for testing until I get 7i74 board delivered?
I am told BISS and SSI though very similar, can't be used interchangeably. I wanted to know which encoder should I order: BISS protocol or SSI? 
I would have three absolute linear encoders, one rotary absolute encoder and one spindle incremental encoder. Is 7i74 better than 7i85 for the my setup?

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30 Nov 2023 14:55 #286872 by pmchetan
The encoder I'm buying has parallel incremental output alongside absolute output. I think I might get the analog output one just to compare it against the performance of the digital output. But I don't know how to configure reading the sine/cosine encoder output to the 7i77 analog GPIO pins. One step at a time for now.

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30 Nov 2023 15:11 #286877 by PCW
Yes, firmware can be made for a 5I25/6I25 + 7I77 that has a BISS or SSI
interface on the 7I77s serial expansion connector.

The 7I77 analog inputs would not be useful for reading a sin/cosine encoder
as they are only read by LinuxCNC at the servo thread rate.

 

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30 Nov 2023 16:35 #286884 by pmchetan
So does flashing 5i25-7i77_7i74 bit file be enough to read from either encoder protocols (SSI/BISS) or is there specific firmware needed for different protocol! I am genuinely thinking of buying the encoder with the protocol default firmware supports.
If there is an available firmware that I can flash to the 5i25/6i25- 7i77 to enable SSI and BISS that I can download and try?

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30 Nov 2023 17:52 #286893 by tommylight
Here is a real life example of using serial absolute encoders with 5i25/7i77/7i74, configs included although not sure if the last production used config is there.
forum.linuxcnc.org/30-cnc-machines/31792...-sbz-130-01-retrofit
there should be firmwares there attached also, gracefully provided by PCW.
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01 Dec 2023 03:24 #286925 by pmchetan

Here is a real life example of using serial absolute encoders with 5i25/7i77/7i74, configs included although not sure if the last production used config is there.
forum.linuxcnc.org/30-cnc-machines/31792...-sbz-130-01-retrofit
there should be firmwares there attached also, gracefully provided by PCW.
 

I got the 5i25_7i77_7i74_ssi6_ss2D . bit and configuration ideas. But can anyone help me with 7i77 firmware to use RS422 port as SSI encoder interface?

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01 Dec 2023 17:06 #286977 by PCW
I can make a 7I77 config for a 5I25/6I25 with one SSI or BISS
interface on the serial expansion, but it may take a couple days
as I am pretty busy currently.

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