FAQ:What is an Indexed CV?
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The NMRA DCC Standard S-9.2.2 allows for more CVs than the 1-1024 limit specified. While many of those CVs are reserved, the NMRA anticipated the need for additional CVs in the future.
NMRA Standard S9.2.2 defined the ranges of 1 – 256 and 512 – 1024 as normal CVs. CVs from 257 to 511 are treated as Paged Index CVs, with the page accessed through CVs 31 and 32, the index registers, or Master CV.
This accomodates up to 65,536 additional pages in that range. The result is a memory space which can accommodate more than a million additional configuration variables.
Indexed CVs give the designers of multifunction decoders more options, especially true of those with sound.
See the article on Configuration Variables for more details.