EOS

[danav]

when i use my ac adapter with my 5d markii, the computer wont recocgnize the camera. happend only with ac not battery. 

[TCampbell]

Are you using the Canon brand ACK-E6 adapter or is this a 3rd party adapter?

I own a 5D II and while I do own the adapter, I haven't tested that particular arrangement.  I _DO_ use the ACK-E6 with my Canon 60Da (astrophotography camera) all the time ... while the USB is connected to a computer which is remotely controlling the image acquistion at the telescope and I have never had an issue.

My 60Da is still in-transit (along with the ACK-E6).  I was at an astronomy event last week.  When I get it back, I'll hook up the ACK-E6 to the 5D II and see if I have the issue you describe.