The same numeric field or length prefix may use different encodings across schemes: packed BCD (two digits per byte), EBCDIC (IBM mainframe charset), or ASCII. A single message often mixes them — numeric in BCD, text in EBCDIC, chip data in binary. This is the classic beginner trap: decoding the whole message with one encoding inevitably misaligns.
Some schemes prepend a self-describing header before the MTI (first byte = header length) carrying total length, format flags, etc.; others have no header and start straight at the MTI. Decide "strip a header or not, and how many bytes" first, or the MTI and bitmap shift as a block.
The standard is a primary plus optional secondary bitmap (128 DEs). Some schemes support a third bitmap (DEs 129–192) for extra private fields. Check bit 1 and bit 65 to know how many bitmaps follow.
| DE | Typical use | Format difference |
|---|---|---|
| DE48 | Additional private data | Some use PDS (tag+len+value subfields) |
| DE55 | EMV chip data | All BER-TLV, but length-prefix encoding/size varies |
| DE62 / 63 | Switching / network private data | Often carry scheme-specific sub-bitmaps; fully private layouts |
👉 The parser tool here supports per-scheme parsing + auto encoding detection. Related: ISO 8583 basics, EMV glossary.