The possible types of conversion are:
You can apply user-defined conversion to selected fields, and leave others to be converted by the CICS® standard conversion program.
For all CICS on System/390 products other than CICS TS for z/OS®, Version 2.2 and later, to take advantage of nonstandard conversion you must provide a single data conversion program named DFHUCNV. This will probably be a customized version of the DFHUCNV program supplied with CICS.
For CICS TS for z/OS, Version 2.2 and later, you can provide either:
If the nonstandard conversion applies only to character data, you may not need to write your own data conversion program. Instead, you could create your own conversion tables for use with the standard conversion program, DFHCCNV. See User-defined conversion tables.
Your user-supplied conversion program must not convert any data that the standard conversion program attempts to convert. Double conversion gives unpredictable results. To ensure that double conversion does not occur, your conversion program must convert only fields defined as DATATYP=USERDATA (see the DATATYP option of the DFHCNV TYPE=FIELD macro).