Is an invalid state in an 8421 BCD counter?

Yes, an invalid state can occur in an 8421 BCD (Binary-Coded Decimal) counter. The 8421 BCD representation can only encode decimal digits from 0 to 9, which corresponds to binary values from 0000 to 1001. Any binary representation from 1010 (A) to 1111 (F) is considered invalid in BCD, as it does not represent a valid decimal digit.