Schematic for the Dell OptiPlex 990 ultra small form factor (USFF) system board, code name Katsiki, revision A00, supplied as a PDF. The USFF board layout differs from the other 990 form factors, so make sure you have the right chassis variant before following it. Use it to trace power rails and standby circuits, check signals between the chipset and the super I/O, and find components around shorted lines. Any PDF viewer opens it, and searching for net names speeds up navigation.
The button opens the matching file on RM Schematics, or a source page if it is not catalogued there yet. The original attachment is from @afsschematicfree. Enginerys does not host this file.
How to open a .pdf file
The standard format for schematics, block diagrams, power-sequence charts and service manuals.