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Joined: Aug 2005
Is it the Mountain Computers one? If so, then it's a lot better than the HP one (it does 80*24 text, for example).
The power input is something like 9-12V AC. The HPIL connectors are obvious :-). And the RCA phono socket on the front is composite mono video, by default at US (NTSC) sync rates, but there's an escape sequence that gives UK (PAL) rates instead.
Try hooking it up and just sending some text to it (as if it was a printer). It should display it.
I have the user manual for The Mountain Computers video interface somewhere, and amazingly it includes the schematic ;-)
IIRC HP actually sold this device at one point since it was so much better than their 82163.