Servo absolute encoder connection

Hi,

I just started with linuxcnc and would be very thankful if you could support me a little bit to get the absolute encoders connected to linuxcnc.
I've got JMC 400w servos with 17bit absolute encoder and JAND4002-20B driver (updated Version of JASD4002-20B).
I already got the servos running with linuxcnc but now I am wondering how to connect the absolute encoders with linuxcnc to avoid homing.
I have a 7i76E mesa card. Do I need an additional card? maybe a 7i94?

The communication is via Modbus RS-485 protocol.
There are two network ports Ch3 and ch4 i think i have to connect the mesa card to one of this ports. That's why I thought I may need an extension card like the 7i94.

I couldn't find the manual online too, I got it from the jmc Support. I would upload it but the file size is too big.
Attached 2 Screenshots aboute the absolute encoder chapter.

Ok I did some more research. I think I can extend my 7i76e with a 7i74 that supports RS-485 on channel 7. Then I can connect my servo drivers all in line to channel 7. Is there anybody who has experience with that? ?

I don't think a 7i74 with RS485 will help you. The JMC servos require Modbus.

Modbus vs RS485:
www.eltima.com/article/modbus-vs-rs485/

I happen to have the same JMC servos with absolute encoders. While I made some progress, I wasn't able to make it work a 100%. I haven't looked at this since early 2020 but maybe food for thought:

forum.linuxcnc.org/38-general-linuxcnc-q...er-position?start=10

I spent quite a few hours on this and while it might be a nice to have feature, I am not sure if the "juice is worth the squeeze".

Try to avoid those cheap USB to RS485/Modbus (without ground pin).

The MB2HAL LinuxCNC module might be the best way to get the data from the drives. You could also use ClassicLadder Modbus.

linuxcnc.org/docs/html/drivers/mb2hal.html

Hi Andy,

don't think Modbus can be used for permanent real-time position information. I believe the best that can be done with these servos here is to pull the drive's absolute position during the LinuxCNC startup phase and then switch to incrementation position from StepGen or a linear scale.

This is at least what I tried last year. I got it working to an extend, but during further testing I noticed that I couldn't find a way to reset stepgen.00.position-fb when I had an e-stop event or I turned the machine off and on again without exiting LinuxCNC. The only way for me to fix this was to close LinuxCNC and restart it.

forum.linuxcnc.org/38-general-linuxcnc-q...tion?start=30#157762

I know of some simple CNC controllers that read Modbus absolute data just during system startup and then use an incremental system for real-time position data.

Note that it is now possible to reset the hm2 stepgen feedback position with a
hal pin or an index input