An implantable medical device (IMD) with internal processor is configured for diagnostic emulation using an external processor coupled to the internal processor through a high speed serial link. The native external processor parallel data and address bus content can be converted to a serial communications stream, sent into the device, converted back to parallel address and data bus formats, and used to drive the device in place of the internal processor. The serial communication allows use of a small number of contact pads, conductors, or feed-throughs, depending on the device. Some devices allow serialized communication through the feed-through typically used for electrical stimulation. The devices can be used to enhance diagnostic testing with capabilities such as faster testing and more realistic testing. The IMD can be a wide variety of implantable devices such as neuro stimulators, pace makers, defibrillators, drug delivery pumps, diagnostic recorders, cochlear implants, and the like. The device can have a bus switch, which when activated, decouples the internal processor, and couples address and data buses containing information and commands provided by the external emulator through the serial communication channel.