DEC H7822 power supply

[Paul Koning]

> On May 8, 2022, at 7:04 PM, Maciej W. Rozycki via cctalk <cctalk at classiccmp.org> wrote:
> 
> On Sun, 8 May 2022, Tony Duell 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.

Why would you expect a transceiver power supply to be isolated from system ground?  The transceiver logic ground (via the AUI connector) is tied to the system ground.  It is the coax that is isolated from the system ground, not the bulk of the transceiver circuitry.

	paul