My Dreamcast can only play backups, not original GDROM. how to disable modchip?

[beterhans]

I have a Dreamcast
US version (Red logo) 110V
I believe it have a mod chip in it.

it can read backup game CDs but can't read original GD-ROM games.

How can I disable the modchip? or is it have a mode like the PS2 modchip?
like original game mode? backup mode? disable mode?

how can I identify what kind of mode chip is in it?

Thanks