Aiken code

The Aiken code (also known as 2421 code)^[1][2] is a complementary binary-coded decimal (BCD) code. A group of four bits is assigned to the decimal digits from 0 to 9 according to the following table. The code was developed by Howard Hathaway Aiken and is still used today in digital clocks, pocket calculators and similar devices^{[citation needed]}.

The Aiken code differs from the standard 8421 BCD code in that the Aiken code does not weight the fourth digit as 8 as with the standard BCD code but with 2.

The following weighting is obtained for the Aiken code: 2-4-2-1.

One might think that double codes are possible for a number, for example 1011 and 0101 could represent 5. However, here one makes sure that the digits 0 to 4 are mirror image complementary to the numbers 5 to 9.

• Excess-3 code
• Gray code
• O'Brien code type I

• White, Garland S. (October 1953). "Coded Decimal Number Systems for Digital Computers". Proceedings of the Institute of Radio Engineers. 41 (10). Institute of Radio Engineers (IRE): 1450–1452. doi:10.1109/JRPROC.1953.274330. eISSN 2162-6634. ISSN 0096-8390. S2CID 51674710. (3 pages)