Err: Cannot Write to LPTx Printing to Parallel Port

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Article ID: 181022 - View products that this article applies to.
This article was previously published under Q181022
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IMPORTANT: This article contains information about editing the registry. Before you edit the registry, make sure you understand how to restore it if a problem occurs. For information on how to do this, view the "Restoring the Registry" online Help topic in Regedit.exe or the "Restoring a Registry Key" online Help topic in Regedt32.exe.
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When you print to a parallel port on a computer running Windows NT, one of the following may occur:

  • After you clear a paper outage or jam on the printer and click OK on the following error message, the same error may reappear until Windows NT is shutdown and restarted:

    Cannot write to LPTx

    where x is the LPT port number.
  • On host-based printers, the printer does not reset properly if you shutdown and restart Windows NT without first actually powering off the computer.
  • When you print with an older parallel port printer, the output has invalid, garbled, or missing characters or nothing prints.


The first two symptoms above are caused by the local port monitor resetting faster than the port can become ready. The printer driver then interprets that the printer is properly reset, which is not the case.

The third symptom happens because the centronics handshake in Parallel.sys is too aggressive for some older devices.


To do a slower handshake to older/slower printers, the following registry entry must be added.

WARNING: Using Registry Editor incorrectly can cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that problems resulting from the incorrect use of Registry Editor can be solved. Use Registry Editor at your own risk.

For information about how to edit the registry, view the "Changing Keys And Values" Help topic in Registry Editor (Regedit.exe) or the "Add and Delete Information in the Registry" and "Edit Registry Data" Help topics in Regedt32.exe. Note that you should back up the registry before you edit it. If you are running Windows NT, you should also update your Emergency Repair Disk (ERD).

The registry entry to enable this feature is

and is a DWORD value "UsePIWriteLoop", which needs to be set to 1.

Be aware that enabling this entry will slow down ALL parallel ports.

To resolve this problem, obtain the latest service pack for Windows NT 4.0 or Windows NT Server 4.0, Terminal Server Edition. For additional information, click the following article number to view the article in the Microsoft Knowledge Base:
152734 How to Obtain the Latest Windows NT 4.0 Service Pack

For your convenience, the English version of this post-SP3 hotfix has been posted to the following Internet location. However, Microsoft recommends that you install Windows NT 4.0 Service Pack 4 to correct this problem.


Microsoft has confirmed that this is a problem in Windows NT 4.0 and Windows NT Server 4.0, Terminal Server Edition. This problem was first corrected in Windows NT 4.0 Service Pack 4.0 and Windows NT Server 4.0, Terminal Server Edition Service Pack 4.


Article ID: 181022 - Last Review: October 7, 2013 - Revision: 2.2
  • Microsoft Windows NT Server 4.0, Terminal Server Edition
  • Microsoft Windows NT Workstation 4.0 Developer Edition
  • Microsoft Windows NT Server 4.0 Standard Edition
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