Ed,
You can use DR-891 with any radio in the capacity similar of that of Digirig Lite.
Nothing should be connected to the red (USB) socket. Black audio socket will facilitate audio and PTT. There are no COM ports added by Digirig and available PTT methods are GPIO3 (AKA RA.Board) or VOX/Tone on right audio channel.
Please see this discussion on Digirig Lite which applies to DR-891 when using with radios other than FT-891.