I purchased a used 5400B last year and it's been in the closet until recently... For $40 extra it came with an ST-2 serial port version. I am on the verge of getting my antenna setup finished and will soon be attempting to get the ST-2 going. I've heard stories about difficulties with USB to serial adapters, so I purchase a serial card to put into my PC and a 15' cable to run to the box for all of $18 on eBay... cheaper than the adapter.
You can watch my QRZ page where I am posting updates about my antenna project and/or remind me in a few weeks to let you know if I got it going. =^D
BTW, the ST-2 is essentially an LVB clone, if that helps any in your research.
73 for now,
Kevin N4UFO
------------------------------------------------------------------ "Control is the need of the fearful mind. Trust is the need of the courageous heart."