I have a Shuttle SG45H7 which I use as my home theatre PC. Currently I have a DVB-T card in the PCI slot and I connect to my TV by HDMI. This PC has an Intel G45 + ICH10 chipset.
Why there is no display from onboard digital output (HDMI) when insert PCI-E interface card?
That is cause by the Intel Hard ware design, when you insert PCI-E interface card, the onboard digital output will not display.
Can the HDMI display when inserting the PCI-E card?
No, it can not. But the D-sub could display in that moment. That is the design limitation of the chipset.
What is the design limitation they refer to, and does it only apply to PCI-E graphics card or will any PCI-E card make the HDMI port stop working? I would prefer to continue using the HDMI connector because I don't then need a separate connection for sound.
This may be a duplicate of this question, although that is a different chipset and talks about DVI not HDMI.