AdS> If one column use WIN1252 and client connect with WIN1252, the server
AdS> send the value without conversion and Jaybird convert to Unicode.

Right.

AdS> If one column use DOS850 and client connect with WIN1252, the server
AdS> convert from DOS850 to WIN1252 and JayBird convert from WIN1252 to
AdS> Unicode;

Right.

AdS> John want to connect using the default character set of the database,
AdS> assuming that all columns is using this charset. Then always only
AdS> JayBird do the conversion.

Not 100% correct. John does not know the default database charset. He wants
first to connect to the database, then somehow read the default charset
(select from rdb$database). After that JayBird performs the conversions.

AdS> Is this correct? Why not connect using NONE charset?

Since there is no way to convert NONE into something else. If you have
database with WIN1252, but your client is NONE, then you have read-only
access. At least so it was till now. Did you change this behavior in new
INTL stuff?

Roman