This diagram illustrate the HMI solution the way I am planning it. I would like one external RS485/UART so I can attach the display to any embedded solution. CAN, USB and SPI Backbone is mandatory. And I think SPI Half Duplex will do fine over max ca 50cm that we talk about here. SPI is also straight forward as it basically only will be a small 50 cent MCU on the add-on modules. I could even use full SPI in this case as we always will be talking to the main MCU and CS pins would solve the issue with knowing what module that I communicate with since these cheaper MCU’s don’t have an id number.