# X PROG 756: Invalid structured field Source: https://wndsoftware.com/mainframe-errors/3270-prog-756 ### X PROG 756: Invalid structured field X PROG 756 means the host sent a Write Structured Field command that contained a structured field the terminal or emulator considers invalid. Usually it is a structured field the terminal doesn't support, or one whose contents don't follow the rules for its type. #### Other names for this code Also written as `PROG 756`, `PROG756`, `X PROG 756`, `3270 PROG 756`, `program check 756`, `PROG 756 structured field`, `WSF error 3270`, `TN3270 PROG 756`. ### 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. A terminal doesn't have to support every structured field. Its reply to a query tells the host which optional functions it supports. If the host sends a structured field with an ID the terminal doesn't know, or one whose contents break the rules for that ID, the terminal stops processing the command. It shows X PROG 756 and locks the keyboard. IBM uses separate codes for some specific structured field problems. A bad length is PROG 759, nonzero reserved fields are PROG 760, and a bad partition ID is PROG 761. PROG 756 covers the general case. ### Common causes - The application sent a structured field the emulator doesn't support. Graphics, image, programmed symbols and explicit partition functions are optional features that many emulators leave out or support only partly. - The application never queried the terminal and assumed features of a real IBM terminal or a different emulator. - A program building structured fields by hand put the wrong ID or wrong data in a field. - The host's terminal definition claims features the emulator doesn't have, so a subsystem or product sends structured fields on that basis. ### How to find the problem - Note which application and function was running. Structured fields are mostly used by graphics and print products, file transfer, query logic at sign-on and applications that build their own data streams. - Take a data stream trace and find the WSF command (X'F3' or X'11'). Walk through each structured field by its length and read its ID. - Compare the IDs with the functions the terminal reported in its query reply. A trace taken from the start of the session will show the query and its reply. - Check the emulator's documentation for the function involved. Some emulators need an option turned on, such as graphics support or file transfer, before they accept the related structured fields. ### How to fix it - For the user, press Reset and Clear and try again. If one function always fails, report it with the function name and time. - Programmers should send Read Partition Query when the session starts and only use the structured fields listed in the reply. - If the application needs an optional feature, enable it in the emulator if the product supports it, or tell users which emulator settings are required. - Fix any structured field the program builds by hand so its ID and contents follow GA23-0059. - Make the host terminal definition match the emulator, so host products don't send features the session lacks. ### Read this error in WND Terminal For a visible X PROG 756 message, [Explain this screen](https://wndsoftware.com/explain-this-screen) can help interpret the screen under IT’s configured AI policy. It does not establish the cause of a rejected host data stream. Preserve the exact status and follow the diagnostic steps in this article. [Try the WND Terminal sample with fictional data](https://wndsoftware.com/demo#launch) · [3270 emulator features and installation](https://wndsoftware.com/3270-terminal-emulator) · [5250 emulator for IBM i](https://wndsoftware.com/ibm-i) ### IBM documentation [IBM Personal Communications documentation](https://www.ibm.com/docs/en/personal-communications/15.0.0). Use the documentation for your installed release. This link opens the relevant IBM reference library or lookup instructions. Match the complete code and your installed product release; it is not a code-specific review. [Report a correction to WND Software](https://wndsoftware.com/contact#message). Technical reviewer: [Albert Joseph](https://wndsoftware.com/authors/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. ### Related codes [X PROG 759 Invalid structured field length Read →](https://wndsoftware.com/mainframe-errors/3270-prog-759) [X PROG 761 Invalid partition ID Read →](https://wndsoftware.com/mainframe-errors/3270-prog-761) [X PROG 758 Invalid mode in Set Reply Mode Read →](https://wndsoftware.com/mainframe-errors/3270-prog-758) [X PROG 750 Invalid 3270 command Read →](https://wndsoftware.com/mainframe-errors/3270-prog-750) [X (input inhibited) Keyboard locked, input inhibited Read →](https://wndsoftware.com/mainframe-errors/3270-input-inhibited) ### Questions #### What is a structured field on a 3270? It is a self-describing block of data sent with the Write Structured Field command. Each one has a length, an ID and data, and it is used for queries, partitions, programmed symbols and other functions beyond a plain screen write. #### Why does PROG 756 happen with one emulator only? Support for structured fields is optional and varies between products. An application that sends a field without checking the query reply works where the field is supported and fails where it isn't. #### Can a user fix PROG 756? Sometimes, by turning on an emulator option the application needs. Otherwise it needs a change in the application or the host definition. [3270 and 5250 screen errors](https://wndsoftware.com/mainframe-errors/terminal-errors) · [X PROG 754](https://wndsoftware.com/mainframe-errors/3270-prog-754) · [X PROG 758](https://wndsoftware.com/mainframe-errors/3270-prog-758) [← All error codes](https://wndsoftware.com/mainframe-errors/)