Keil Logo

C51: Using C51 Version 7.50 with a C51 Version 8/9 PSN

Information in this article applies to:

  • C51 Version 8.xx or later with node locked license (PSN/LIC)


I purchased a C51 compiler license and received C51 Version 8/9 and a Product Serial Number (PSN). The project I'm working on requires the C51 Version 7.5 compiler. 

How can I install C51 Version 7 instead of C51 Version 8/9?


To downgrade C51 Version 8/9 to C51 Version 7.50...

  1. Install C51 Version 8/9 from the CD-ROM or download file and add your license as explained in the Licensing User's Guide.
  2. Download the file attached to this article using your License ID Code (LIC).
  3. Unzip and run the file inside the archive. Select exactly the same folder that you specified for the PK51 or CA51 installation.
  4. Open TOOLS.INI (located in your installation folder) with a text editor.
  5. Within the [C51] section, look for the line LIC0=XXXXX-XXXXX-XXXXX-XXXXX-XXXXX-XXXXX.
  6. Copy that line and change LIC0 to SN in the copied line so that it reads: SN=XXXXX-XXXXX-XXXXX-XXXXX-XXXXX-XXXXX
  7. Save and close TOOLS.INI

C51 Version 8/9 is now downgraded to C51 Version 7.5.


  • This method only works for Single User Licenses. It does not work when you are using other license types, i.e. Keil Floating User Licenses.


Request the files attached to this knowledgebase article.

Last Reviewed: Friday, January 8, 2021

Did this article provide the answer you needed?
Not Sure
  Arm logo
Important information

This site uses cookies to store information on your computer. By continuing to use our site, you consent to our cookies.

Change Settings

Privacy Policy Update

Arm’s Privacy Policy has been updated. By continuing to use our site, you consent to Arm’s Privacy Policy. Please review our Privacy Policy to learn more about our collection, use and transfers
of your data.