I found this while digging through a surplus store.
AT&T external SCSI
drive model DM/300S. It's a white metal box about 11" square and 4" high.
Does anyone know what machine/system it's for or the specs for it? It has
two round co-axial connectors on the back marked "Power" and
"Control".
Anyone know what they're for? It has a socket for an AC power cord so the
"Power" connection isn't for powering the drive.
Sounds suspiciously like it's for a 3B2.
The 3B2 used an interesting daisy-chained soft-power scheme. When the main
system was powered up, it would put out 5V on a coax port on the back. The
5v would close a relay on the power supply of the expansion box (in lieu
of a switch) and turn on power to the expansion box.
I'm guessing here, but I assume that one of the ports is the 5v in and the
other would output 54v to power up the next peripheral in line.
<<<John>>>