X PROG 758: Invalid mode in Set Reply Mode

What it means

Write Structured Field (WSF) is the 3270 command for anything more complex than painting a screen. Behind the command comes one or more structured fields. Each one starts with a two-byte length, then an ID that says what kind of field it is, then its data. Common outbound structured fields include Read Partition (used to query the terminal), Erase/Reset, Set Reply Mode, Load Programmed Symbols and Outbound 3270DS, which carries an ordinary screen write inside a structured field.

Set Reply Mode controls how the terminal formats what it sends back when the host reads the screen. There are three modes. Field mode, the default, reports fields with their basic attributes only. Extended field mode also reports each field's extended attributes, such as color and highlighting. Character mode also reports attributes set on single characters, for the attribute types the host lists in the same structured field.

The structured field names a partition, then a mode byte, and in character mode a list of attribute types. If the mode byte isn't one of the defined modes, or asks for something the terminal can't do, the terminal rejects the command. It shows X PROG 758 and locks the keyboard.

Most applications never change the reply mode, so this code is rare. When it appears, it points to an application or product that uses Set Reply Mode on purpose.

Common causes

• The mode byte holds a value other than the three defined modes, often because the program built the structured field with the wrong offsets.
• The application asked for character mode or extended field mode on a terminal that doesn't support it.
• The structured field names a partition and mode combination the terminal can't apply.
• The application assumes support it never checked in the Query Reply.

How to find the problem

1. Note the application and function. Set Reply Mode is used by software that needs to read back colors, highlighting or character attributes from the screen.
2. Take a data stream trace and find the Set Reply Mode structured field (ID X'09') inside the WSF command. Read the partition byte, the mode byte and any attribute type list after it.
3. Field mode is X'00', extended field mode is X'01' and character mode is X'02'. Any other value explains the error by itself.
4. Check the terminal's Query Reply to see whether it supports the mode the application asked for.

How to fix it

• For the user, press Reset and Clear, then retry. Report it with the application name if it repeats.
• Correct the mode byte or the offsets in the code that builds the structured field.
• Query the terminal at the start of the session and fall back to field mode when the terminal doesn't support the other modes.
• If the data stream is valid and the terminal claims support, take the trace to the emulator vendor.

Technical reviewer: Albert Joseph, WND Software.

Last reviewed 2026-10-18 by the WND Software team. Written in our own words. For the full reference, check IBM’s 3270 or 5250 display documentation.