ISO 8583 · Data fields

Field 101File Name

pyiso8583 starter profilesIndependent explanation

What this field describes

This variable length field carries a file name in the selected profiles.

Read it as a protocol value. It does not authorize reading or writing a local filesystem path.

Compare the two profiles

Container settings from the pinned pyiso8583 source.
Setting	`default`	`default_ascii`
Value encoding	ASCII	ASCII
Length limit	Up to 17 bytes	Up to 17 bytes
Prefix	2 ASCII bytes	2 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 prefix2 bytesASCII length

Field 101≤ 17 bytesASCII value

Read the 2 prefix bytes first, then read that many value bytes. Reject a length that exceeds 17 or the bytes remaining in the message.

Find it in the bitmap

Bitmap: Secondary
Position: 37
Byte: 5
Bit in byte: 5

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.