No HDMI from Radeon 5870 eyefinty 6 edition

I have recently got a Sapphire Radeon HD 5870. It in in my Asus M4A785TD-V EVO motherboard. I get video out throught the mini display port to DVI adapter. The HDMI does not work throught the mini display port to HDMI adapter. It can't be detected. The only way i can get the HDMI to work is to Remove the working DVI adapter then plug the HDMI to the same port the DVI came out of. I dont think that this is a videocard issue because the onboard video on my motherbaord stopped being able to detect HDMI on startup a few months ago.