To get a HIGH(1) output from a NOR gate, all the inputs must be LOW(0). If any of the inputs are HIGH(1), the output will be LOW(0). Looking at the truth table, only TRUTH[0] fulfill this requirement. This means that TRUTH[1] to TRUTH[7] must be LOW(0) and TRUTH[0] must be HIGH(1), giving the hex value 0x01 to put into the TRUTHn register.
Figure 1. NOR Gate
