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You can connect TTL level encoders directly to 7C81 I/O pins
_but_ I would at the minimum add say 220 Ohm resistors in series
with each signal to protect the inputs from negative input spikes.
(encoder signals tend to pick up motor drive noise so are fairly hard
on unprotected inputs)

I'll answer with experience of using Aliexpress Omron type encoders www.aliexpress.com/w/wholesale-omron-rotary-encoder.html , which are about less than a tenth of the price of the ones you mentioned, it's nice if you mention parts you can provide a link, saves time trying to search and ensures the correct component is being discussed.

With an optocoupler you will need to ensure the optocoupler is fast enough and the encoder can drive the optocoupler's led hard enough.

If the encoder is true TTL output or open collector output it should be fine.

Personally I've just used simple buffers, 74hc540/541, as I have a couple of tubes of them.

Another option would be to use the differential output of the encoder and connect that to a differential line receiver, such as www.futurlec.com/Linear/26LS32pr.shtml.

The schematics for the 7c81 are available, forum.linuxcnc.org/media/kunena/attachments/481/7c81sch.zip , one of the sheets shows the input circuitry.

No doubt Pete from Mesa will chime in.

Hello

I have a question about connecting the encoders in the set:

RPI4 + 7C81 + Mach5 3-axis board.

Since the 7c81 documentation says about a 5V tolerance (inputs), can I connect the encoders directly to the 7c81, or is it better to protect them with an optocoupler?

Will the encoders:
ROD426
ROD420
be suitable?

Thanks in regards, 
Michal

After six weeks of doing everything wrong a few times and being graciously corrected by everyone in the community my franken-build Pi 5 debian is working very well.

I'm about to start clean and do everything as has been documented in this thread without my own improvisations which I think got me little except the motivation to learn everything necessary to fix all the things I broke.

I wanted to make sure I'm starting with the latest of Rod's builds and I think it is this from the Google-Driver:

   rpi-5-debian-bookworm-6.6.54-rt39-arm64-ext4-2024-11-06-2037_qtpyvcp_stable_networkmanager.img.xz

The one on the Downloads page is still:

   rpi-5-debian-bookworm-6.1.61-rt15-arm64-ext4-2023-11-17-1520.img.xz

I'm pretty sure I want the one from Nov 2024 instead of Nov 2023.

Is there an even newer one?