ISO 8583 · Data fields

Field 104Transaction Description

pyiso8583 starter profilesIndependent explanation

What this field describes

This variable length container holds a transaction description in the selected profiles.

Keep descriptive text separate from transaction identifiers and status fields. Check character encoding and length before transmission.

Compare the two profiles

Container settings from the pinned pyiso8583 source.
Setting	`default`	`default_ascii`
Value encoding	ASCII	ASCII
Length limit	Up to 100 bytes	Up to 100 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 104≤ 100 bytesASCII value

Read the 3 prefix bytes first, then read that many value bytes. Reject a length that exceeds 100 or the bytes remaining in the message.

Find it in the bitmap

Bitmap: Secondary
Position: 40
Byte: 5
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.