X PROG 751: Invalid character set in an order

What it means

A 3270 data stream can set extended attributes for a field or for single characters. Three orders do this. Start Field Extended (SFE) starts a field and gives it attributes. Modify Field (MF) changes the attributes of a field that already exists. Set Attribute (SA) sets an attribute for the characters that follow. Each attribute is a pair of bytes, a type and a value. One of the types is the character set.

The character set value tells the terminal which set of symbols to draw. The default set is always there. Others are optional. Examples include the APL and text set, a double-byte set for languages such as Japanese, and programmed symbol sets that the host loads into the terminal. If the value names a set the terminal doesn't have, or a set that was never loaded, the terminal can't draw the data. It rejects the write, shows X PROG 751 and locks the keyboard.

Common causes

• The application uses the APL or text character set, or a double-byte set, and the emulator session isn't set up for it.
• The application refers to a programmed symbol set it expects to have loaded earlier, but the load didn't happen in this session, for example after a reconnect.
• The application was written for a terminal model with more features and doesn't check what the current terminal supports.
• A program building SFE, MF or SA orders by hand put a wrong value in the character set position, or got the attribute pairs out of step.

How to find the problem

1. Write down the screen and transaction. A PROG 751 that happens on one screen only points to a field on that screen.
2. Take a data stream trace and look for SFE (X'29'), MF (X'2C') and SA (X'28') orders. The character set attribute type is X'43'. Check the value that follows it.
3. Compare that value with what the terminal reported in its Query Reply. A terminal lists the character sets it supports there, and a session that never sent a query can't be relied on to have more than the default set.
4. Check the emulator session settings for APL, DBCS or code page options. A host code page set for one language and a session set for another can also lead the application to ask for a set the session lacks.

How to fix it

• For the user, press Reset, then Clear, and try again. If the screen needs a special character set, ask whoever manages your emulator to enable it for your session.
• Programmers should send a Read Partition Query when the session starts and only use the character sets listed in the reply.
• If the application uses programmed symbols, load them (Load Programmed Symbols) before any screen that uses them, and again after a reconnect.
• In CICS, check whether the terminal definition (TYPETERM) claims programmed symbols or DBCS support that the emulator doesn't actually have. BMS will use those features if the definition says they exist.
• If the value in the trace is plainly wrong, fix the code that builds the attribute pairs.

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.