In the past the typical debug backdoor was a RS232 interface. I have for instance a BlueWave multi-DSP VME board, which has beside the VME and an ethernet board an RS 232 for console output. The reason RS232 was chosen for this purpose is obvious, cheap, small, available on most PCs, easy to implement and in a way foolproof (or at least problems such as wrong pinout can be easily fixed).
I would expect that this role will is been taken over by USB now. The question is however, what type of service is used. Is there something established, or is this not necessary? I am aware that I can do a serial interface, jtag, etc. through USB, but this variety would counter the notion of "simply working without special SW" that the RS 232 provided.
What do you think?