I've temporarily owned such a system a few years ago. The pictures on Tekwiki show that system. But it's not that easy. On top you'll need a terminal and a compatible signature analyzer. And some components like the signature tables (and maybe the diagnostics ROM) have to be matched to the hardware revision of your 7854.
The diagnostics ROM board also contains some stand-alone diagnostics routines which are helpful for calibration and some digitizing problems.
I've uploaded the involved firmware roms and scans of the manuals to tekwiki. I think the vintageTEK museum is still selling a clone of the diagnostics ROM board.
But in my opinion it's way easier to diagnose the system "by hand".
Holger
