ISO 8583 · Data fields

Field 48Additional Data · Private

pyiso8583 starter profilesIndependent explanation

What this field describes

The starter profile provides a variable length container for private additional data.

Private data requires an agreed schema between the systems. No universal subfield meaning is assigned here.

Compare the two profiles

Container settings from the pinned pyiso8583 source.
Setting	`default`	`default_ascii`
Value encoding	ASCII	ASCII
Length limit	Up to 999 bytes	Up to 999 bytes
Prefix	3 ASCII bytes	3 ASCII bytes

Length limits describe encoded values. They exclude any length prefix. The profiles can be customized; these settings do not establish the required fields or accepted values for a live connection.

Read the field boundary

Length prefix3 bytesASCII length

Field 48≤ 999 bytesASCII value

Read the 3 prefix bytes first, then read that many value bytes. Reject a length that exceeds 999 or the bytes remaining in the message.

Find it in the bitmap

Bitmap: Primary
Position: 48
Byte: 6
Bit in byte: 8

Positions start at one. Bits are counted from the most significant bit of each bitmap byte. This calculation locates the field’s presence indicator, not its value.