Raspberry Pi 4 with Mesa 7c81

Thank You very much

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

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.

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)

Thanks,

Maybe a stupid question, but how do you use the relay on those cheap MACH3 BOBs with the 7c81?

For the inputs to work I need to enable the pullups on the 7C81.
But enabling the pullups makes the relay close as soon as the BOB is powered.

Ideally I would want the relay to be open until linuxcnc is started and I tell linuxcnc to close the relay.

But the relay closing by default due to the pullups makes it unusable, or did I miss something?