DEC H7822 power supply

Tony Duell ard.p850ug1 at
Sun May 8 09:25:54 CDT 2022

On Sun, May 8, 2022 at 1:02 PM Maciej W. Rozycki via cctalk
<cctalk at> wrote:

>  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
> wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
> across the suspicious connections as any reference to ground may not be
> indicative.
>  This voltage is also present with the H7819 PSU.  It's not clear to me
> what it is used for.

It's only a guess (this is rather more modern than the DEC hardware I
have), but it may be a supply for a thinwire ethernet transceiver
circuit. This would be insolated from system ground of course, and
I've seen something similar in an X-terminal.


>   Maciej

More information about the cctalk mailing list