ISO8583 - Encoding method for the Numeric part of the AlphaNumeric field type

Question

Should I use BCD encoding for the numeric part in the Alphanumeric field while the message encoding is BCD instead of ASCII?
For example, should I encode "12AB" to x31 x32 x41 x42 (all ASCII) or x12 x41 x42 (BCD and ASCII) ? and if it's former, then what's the purpose of Alphanumeric? Why not just use Alpha?

Answer 1

BCD encoding used usually for numeric-only fields like amounts, dates, sometimes Track2 where D used as separator and feet BCD encoding. Alphanumeric fields stay in ASCII.

And even if your message specification force to use BCD for the alphanumeric fields it is expected to use half of byte (nibble) for each ASCII character. Then the supported letters could be only A,B,C,D,E or F.

For you example "12AB" ASCII expected to be BCD encoded as 0x12 0xAB.