I think there is some confusion here carried over from the other thread.
The A/D conversion is done by the computer interface. For example, if you are using an audio interface that hooks into the computer via Firewire or some other digital connection, that interface is doing the analog to digital conversion.
Perhaps the problem was in the language that was used. These days ppl just call them an audio interface but they are basically the A/D converters that we were talking about in the other thread.
Check out my reply. Your explanation about outboard gear was correct and that's how it's done. The data is being converted from D to A when it comes out of the computer and then from A to D when it goes back into the computer.