I have designed quite a few add-on cards for mother boards including PC mother board, Raspberry Pi and quite a few custom boards. I have worked on 8bit,16 bit and ARM core 32 bit processors from ATMEL, TI, NXP, Microchip, Analog Devices, Silicon Labs, Dialog semiconductor, among others.. I also have extensive experience on RS 232, RS 485, RS 422, CAN, I2C, SPI and Ethernet communication interfaces
I have extensive design experience in developing firmware board support package and complete application firmware
The final price will be decided on the complexity and scope of work.
You have mentioned need for piggybacking a second board. Will need more clarification on the same. Also, RS 485 needs transmit enable pin. Do you want it to be Auto transmit enabled?