STM32 connection MESA 7I76E

your options are either

* develop smart serial firmware for your STM32 board. look at stmbl code for how it could be done on a STM32F405, should be similar enough. protocol should be documented in 7i76 docs.

* connect encoders to encoder input on mesa, connect STM32 via USB to computer, implement USB HID keyboard

* connect everything to STM32 and implement USB HID pendant interface (like those chinese xhc06 et al)

* use modbus on STM32, write linuxcnc component to talk to your STM32 via USB serial interface. usually RS485 is used for modbus. you could also use modbus via "pktuart" on mesa, but that code is probably not finished and on the bleeding edge, so not recommended for newbies. there is modbus interface generator in linuxcnc source you could use.

If I were you, i would connect encoders directly to MESA 7i76 and implement USB-HID for keyboard.

what exactly is the encoder for? is it an interface encoder like pendant or a machine/motor encoder?
How far away is the stm32 from the mesa and the pc? I think there is mesa firmware for spi master
Do you only need it to do the encoder thing? you could output encoder a b z from the stm32

If you are doing some kind of screen thing and you want to convert touchs to jog pulses?