address came about. Why is “4-digit” incorrect? Because there
are at least 871 3-digit addresses that fall in to the long address
realm. Addresses from 128 to 999 must be long addresses, but
only have three digits. See figure 2 for an example.
However, the designers didn’t make it as simple as it could
have been and put two digits in CV 17 and the other two dig-
its in CV 18. No, there is a complicated formula for what is
stored in the two CVs so the decoder knows its name. NMRA
Recommended Practice (RP) 9.2.2 allows long addresses from
0 to 10239. I don’t know of a system that currently addresses
locos above 9999, though. More on this later.
The decoder needs to know what name (address) to respond
to: the short address in CV 1 or the long address in CVs 17
& 18. This is sort of like my Tom and Thomas analogy above.
2: Loco with cab address of 268 – this will be a long address
on any system, even though it has only 3 digits.
2
