Hi,
This HP Z840 Workstation supports 2 Intel Xeon Processors E5-2600 v3/v4 family and
hence both E5-2650 v3 CPU and E5-2690v3 CPU are supported with this model with the stock PSU.
So if you are using 2 processors, please make sure that the second processor is the same as the first.
Also, this unit does not have integrated graphics, so please make sure that the graphic card is used for the unit to boot.
You can check the following link to get the best or optimized performance for memory configuration, recommended graphic card and memory load order:
http://www8.hp.com/h20195/v2/getpdf.aspx/c04400043.pdf?ver=2
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Regards,