X PROG 750: Invalid 3270 command

What it means

Every piece of output a host sends to a 3270 display starts with a command. The command tells the terminal what to do with the rest: write to the screen, erase and write, erase the input fields, read the screen back, or process structured fields. Each command is a single byte. There are two sets of codes, one used on SNA sessions and one used on non-SNA attachments. Erase/Write, for example, is X'F5' in the SNA set and X'05' in the local set.

When the first byte isn't a command the terminal recognizes, it can't make sense of anything after it. It throws the output away, shows X PROG 750 in the operator information area and locks the keyboard. The PROG prefix means a program check. The terminal is saying the host program sent something wrong. It isn't your typing.

Because the whole write is rejected, the screen usually stays as it was before, or stays blank if it was being cleared.

Common causes

• Data meant for another kind of device reached a display session, for example printer data sent to a 3270 display LU.
• An application that builds its own 3270 data stream put the wrong byte first, or left out the command and started with the WCC or with text.
• The output was split or shifted somewhere along the way, so the terminal took a byte from the middle of the data as the command. Errors in TN3270 record handling in a server or emulator can do this.
• The application sent a command the terminal doesn't support in this session, for example because it assumed features the emulator never reported.

How to find the problem

1. Note the transaction or screen that was running when PROG 750 appeared, and whether it happens every time. A repeatable error points to one program.
2. Take a data stream trace in the emulator, or a packet capture of an unencrypted TN3270 session, and look at the first byte of the failing inbound record. Compare it with the command table in the 3270 Data Stream Programmer's Reference (GA23-0059).
3. On the host, a VTAM buffer trace for the terminal's LU shows exactly what was sent. In CICS, CEDF shows the SEND command and its data before it goes out.
4. Check which LU and logmode the session uses. A display session bound with a printer or non-3270 logmode will receive data it can't handle.
5. If every application gives PROG 750 on one emulator only, suspect the emulator or TN3270 server and check for fixes.

How to fix it

• For the user, press Reset, then Clear, and start the transaction again. If it comes back, write down the transaction and time and report it.
• Programmers writing raw 3270 output should make sure the data starts with a valid command for the session type, followed by the WCC. In CICS, let CICS supply the command and WCC through SEND options such as ERASE and CTLCHAR rather than putting them in the data.
• Send printer output to a printer LU and display output to a display LU. Check the terminal definitions and logmodes if the two have been mixed up.
• Before sending structured fields or other optional features, query the terminal (Read Partition Query) and use only what it reports.
• If a trace shows the data was correct when it left the host, open a problem with the TN3270 server or 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.