Fix an HMI help file that will not open

What Windows is blocking

When a browser saves a file, Windows attaches an alternate data stream named Zone.Identifier that records where the file came from. The mechanism is known as Mark of the Web, and the documented stream value ZoneId=3 marks a file downloaded from the internet zone. The compiled help viewer that opens .chm files applies that mark: the window and the topic tree render, and the right pane stays blank or prints a page error. The viewer's trust rules reach further than the flag alone; the vendor's own documentation on restricting HTML Help functions to approved folders shows how far they go. The PowerShell Unblock-File cmdlet removes the Zone.Identifier stream, which is the whole fix for the flag. One more fact helps on the plant floor: the stream is an NTFS feature, so a copy made to a FAT32-formatted USB stick arrives without the flag.

Symptoms and what each points to

Read the symptom before touching anything. Each one has one cause and one fix.

Two attempts change nothing, so skip them. Re-downloading produces a fresh copy with the same flag, because the flag comes from the act of downloading. Reinstalling the panel software leaves the separately downloaded help file wherever it sits.

Related failures live elsewhere on this site: a replaced help file that serves an Action cancelled page is fixed by the Action cancelled fix for replaced HMI help files, and a project backup restored from CD-R that will not open is fixed by clearing the read-only flag on an HMI project backup. Both are file-level states, like the flag here.

Fix it in order

1. Read the extension. Turn on file name extensions in Explorer and look at the file. A .chm file follows the rest of these steps. A .hlp file has no viewer on Windows 10 or 11, so go straight to the vendor for a current format.
2. Unblock the archive before extracting. Right-click the downloaded zip, choose Properties, tick Unblock at the bottom of the General tab, and click OK. Unblock-File does the same job from PowerShell. Unblocking first means every extracted file lands clean; unblocking files one by one after extraction misses the companion files that compiled help needs.
3. Extract everything to a short local path. Use Extract All and pick a folder such as C:\Help. Keep the full path under the Windows limit of 260 characters, because a deeper folder can break the viewer's links to its own pages. A network share brings the blank pane back even when every file is clean.
4. Unblock a file that is already loose on disk. Work the three commands in order:

Get-Content .\yourfile.chm -Stream Zone.Identifier
Unblock-File -Path .\yourfile.chm
Get-Item .\yourfile.chm -Stream Zone.Identifier

The first command prints the flag. Unblock-File removes the stream. The last command then reports that the stream is missing, and that report is the confirmation you want. yourfile.chm stands in for your real file name.

5. Reopen it clean. Close every open help window, then double-click the file from the extracted folder.

Where a file sits deciding whether content shows is a repeating theme on the factory floor. The same class of problem breaks graphics after a migration, and HMI image path troubleshooting after migration walks that case.

Example: a blank pane on the panel-engineering laptop

The laptop pulls the software and the help archive into the Downloads folder. The archive is extracted to the desktop, yourfile.chm is double-clicked, the topic tree fills in, and the right pane stays blank. Read the flag:

Get-Content .\yourfile.chm -Stream Zone.Identifier

[ZoneTransfer]
ZoneId=3

ZoneId=3 marks a download from the internet zone, so the flag is the cause. Unblock the archive, extract all of it again to C:\Help, and read the flag on the new file:

Get-Content C:\Help\yourfile.chm -Stream Zone.Identifier

The command now reports that the stream is missing, because the archive was unblocked before extraction. Double-click the file, click three topics in the left tree, and each one renders text and images in the right pane. The full path C:\Help\yourfile.chm is 19 characters long, well inside the 260-character limit, so nothing else stands in the way. The file's content never changed from one folder to the next. The only difference is the missing stream, and with it gone the viewer renders every page.

Checks that confirm the fix

• Click three topics in the left tree. Each one must render text and images in the right pane.
• Open the index or search tab and run one lookup. A file that is still flagged tends to show the tree and return nothing.
• Open the help from the software's Help menu. If the menu route fails while a direct double-click works, the file sits in the wrong folder for the menu link, and moving it to the location the download page names fixes the menu.
• Finish with the stream check: Get-Item C:\Help\yourfile.chm -Stream Zone.Identifier must report that the stream is missing.

Recommendation and repeat prevention

Recommendation: unblock the downloaded archive before extracting, and keep the extracted help file on a local drive under a short path such as C:\Help, inside the 260-character path limit, on every workstation that needs it. Unblock a loose file directly with Unblock-File when it is already extracted, and expect to repeat the whole check after a laptop rebuild, because the rebuilt machine downloads the file again with a fresh flag.

Keep it from coming back:

• Unblock the archive first, every time. File-by-file unblocking after extraction misses the companion files compiled help needs.
• One local copy per workstation. A shared engineering drive brings the blank pane back through the network-path restriction.
• On a plant PC managed by group policy, the Unblock checkbox and the unblocking command can be unavailable. That restriction is deliberate. Agree a local folder for help files with your IT group, keep unblocking to machines you administer, and leave the registry alone.

Questions