This paper discusses the possibilities of using certain constant‐weight block codes in optical data links and in optical ring networks. The codes that appear to be particulary attractive have exactly as many 1s as 0s in each code word, so there is no penalty for ac coupling. Codes can be simple, such as the Manchester code. Or they may be more complicated and support control channel signaling, error detection, and forward error correction. This paper bridges the gap that sometimes exists between the mathematically “ideal” block codes and the typical requirements of particular applications. It presents a variety of constant‐weight codes of increasing length, including a code of length 16 that is being implemented for a very high‐speed, ultra‐reliable, ring data network.