Can Eidolons stay conscious until they are killed?

[Ferio]

I've looked everywhere and it's a bit confusing. I see this rule:

"Eidolons are treated as summoned creatures, except that they are not sent back to their home plane until reduced to a number of negative hit points equal to or greater than their Constitution score."

and this one:

"Whenever the eidolon takes enough damage to send it back to its home plane, the summoner can, as a free action, sacrifice any number of hit points. Each hit point sacrificed in this way prevents 1 point of damage done to the eidolon. This can prevent the eidolon from being sent back to its home plane."

This would lead me to believe the Eidolon can fight as per normal when he goes negative hit points without falling unconscious until he reaches his negative con in HP. At THAT point, the summoner can give his hp to keep it up.

Am I seeing this wrong or can Eidolon's fight normally all the way to their negative con score?

[wraithstrike]

There is no rules exception to keep them fighting at negative hit points so they can be unconscious, and still remain on the battlefield.
I only think a summoner would allow this if he thought the eidolon was going to get healed soon or if the combat was over.

[Maezer]

The Eidolon falls unconscious just like everything else. It makes Diehard a fairly useful feat for Eidolon's though.