A method includes, storing a set of valid codewords including: a first valid functional codeword representing a functional timeout state of a second controller; a first valid fault codeword representing a fault timeout state of the second controller and characterized by a minimum hamming distance from the first valid functional codeword; a second valid functional codeword representing a functional state of a system; and a second valid fault codeword representing a fault state of the system; in response to detecting receipt of a safety message from the second controller within a predefined time quantum, storing the first valid functional codeword in a first memory; in response to detecting a match between contents of the first memory and the first valid functional codeword, outputting the second valid functional codeword; in response to detecting a mismatch between contents of the first memory and every codeword in the first set of valid codewords, outputting the second valid fault codeword.