You must have the MDR and the associated driver for any module that interfaces directly with the computer.  MDRs for slave modules (e.g., CAMAC, VME, PXI, etc.) that only communicate with a controller module via the instrument bus do not require an MDR.  If no MDR is available for the module that you want to use, just leave the slot blank and enter the  CAM (common addressing model)  address  by hand.  MDRs are just text files and are generally easy to write.  Associated drivers must often be native code modules.  The MDR is used to locate, load and initialize the low-level driver for the module.