Chapter 22 · DMX512-A 229 E NHANCED F UNCTION One of the reasons the 2004 DMX512-A standard took so long to complete was the conten- tious issue of whether to expand the standard to encompass a large amount of noncompliant work that had been done over the years, or to restrict the standard and exclude that equipment. The tightening approach (which I favored) eventually won out, although motions had to be made, and votes taken to essentially force compliance with the existing DMX standards and not risk damaging any legacy equipment. However, the new standard does embrace and specify enhanced function that will not damage legacy equipment and defined four enhanced function topologies, numbered 1­4. · Enhanced Function 1 allows half-duplex EIA-485 signals on the primary data link and other data to return on the same (primary) data link, provided that this is done using a regis- tered alternate start code. No functionality is specified for the second data link. RDM uses Enhanced Function 1. · Enhanced Function 2 allows unidirectional DMX data on the primary link and 485 unidi- rectional data flowing the opposite direction on the secondary link. · Enhanced Function 3 is unidirectional DMX data on the primary link and half-duplex 485- compliant signals on the secondary link. · Enhanced Function 4 is half-duplex 485-compliant signals on both pairs, and the return signal is on the primary data link using a registered alternate start code.