Q.Write the table of 2-4-2-1 code and show that it is a self complimenting code.

A digital code is a systematic way of representing numbers, letters or other information using groups of binary bits. Codes are used because digital systems can only handle 0s and 1s, and a well-chosen code makes storage, transmission, arithmetic and error detection easier and more reliable.

Codes are first classified by whether the bit positions carry a fixed value. In a weighted code, each bit position has a definite positional weight and the decimal value is the weighted sum of the bits. The 8-4-2-1 BCD code and the 2-4-2-1 code are weighted. In a non-weighted code, the bit positions have no fixed weights, so the value cannot be found by a simple weighted sum. The Excess-3 code and the Gray code are non-weighted.

A special group is the self-complementing codes, in which the 9's complement of a decimal digit is obtained merely by complementing (inverting) each bit of the code word. The Excess-3 code and the 2-4-2-1 code are self-complementing, which is very convenient for building subtractors and BCD arithmetic circuits.

Other useful categories include sequential codes, where each successive code word is one binary count higher, and alphanumeric codes, which represent letters and symbols as well as digits. Knowing how to place a code into the right category — weighted, non-weighted, self-complementing — is a frequent one- and two-mark question in the 2nd PUC Electronics examination.