content: Even then, to replace the firmware you first have to show the old firmware that the new firmware is correctly signed by the original manufacturer.  So, say that you perfectly know all the chips in the hardware (which might be impossibile because sometimes those chips are developed under strict NDAs that don't let the manufacturers to share any info with you) and you perfectly know how to program the main processor (which, btw, might be proprietary and not have a public instructions set and/or datasheet and/or manual) to talk to each one of them... you still can't update the old firmware with your own because you are missing the signing keys that only John Deere has. When you attempt to do it the microcontroller will refuse your firmware.