I also have acquired a Supra Atari HD, which came with a 105 Mb Quantum SCSI drive that was already formatted and loaded with Supra's utilities, so that it would boot. One of the utilities was a clock-setting routine. When I ran it I came to the conclusion that the Supra unit must have a RTC itself, because there were three clocks shown (System Keyboard and Supra), all with the wrong time.
So I opened up the Supra drive to see what was the RTC, and I found the twin of the above board, this one all connected. Referring to the attached picture you can see that the 50-pin header is cabled to the drive, the 19-pin d-sub is for the expansion or second ACSI drive and the 26-pin header goes to the 25-pin d-sub on the case. J2, which is empty in the above board, has a right-angle header to which is affixed the Atari ACSI cable's input.
The RTC has a dead battery but that is solvable; Maxim replacements are still available and the chip can be (carefully) modified to fit a new external battery. The power supply visible below quite likely puts out +5VDC although I have yet to measure it and jumper JP1 is pinned the left- or forward-two pins connected. For this reason I do not believe JP1 is a battery input.
More investigation is necessary to figure out what pin on the SCSI out port mates with what pin on the 26-pin header, and to verify voltage, but ZI thought I would post these findings now anyway. More to come.
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