DEC 874-A Power Conditioner

[Tony Duell]

> or 'bay' between the three uprights has a 874-A power conditioner / 
> power-strip unit that I'm trying to understand.  I haven't been able to 
> find any references to this component.  Though I could just replicate 
> the configuration that it had before being relocated (switches, wiring, 
> etc.), which I have plenty of photos of,  I'd like to understand this 
> compenent better.
> 
> Since I have a double-wide cabinet, I've got two of these units that are 
> linked together with a 3-conductor cable, each end plugged into one of 
> the four "Power Control Bus"-labeled sockets.  One 874-A also has 
> another connection from a second "PCB" socket to the the 11/23+ 
> processor box.  My guess is this line somehow is controlled by the front 
> panel AUX ON/OFF switch.
> 
> Also, each of the 874-A units has a small 3-position toggle switch: 
> 'Remote On', 'Off', and 'Local On'.  Before I go much further with 
> experimenting on my own, I wonder if anyone here knows these units and 
> can offer some info about how these units tie together and what settings 
> do what.  Comments?


All thse DEC power controllers are basically the same, although the 
internal circuitry differs somwahat. 

They contain a large relay (contactor) that controls the mains to the 
switched socket outputes on the unit. This is controlled by some 
simple-ish electronics in the unit (IIRC, the 861 controller used a 
differentially-wound reed relay!) Anyway, the 3 positions of the toggle 
switch mean 

Off : The contactor is always off, no matter what happens on the power 
control bus

Local On : The contactor pulls in, it can be turned off by the shutdown 
line on the power control bus, though.

Remote On : The contactor is controlled by the power control bus

The 3 wires in the power control bus are : Ground, Ground-for-on (which 
is how you turn on the contactor in the Remote-On mode), and 
Ground-for-off (which is a shutdown line that will turn off the contactor 
no matter what it telling it to be on). There is no latching associated 
with this, toy can control the contactor using the ground-for-on line only.

Typically, overheating-detection thermoswitches are connected between the 
ground wire and the ground-for-off wire. If any part of the machine 
overheats, the power controllers turn off the power to everything.

The frontpanel switch is wired between the ground and ground-for-on 
wires. Thus the frontpanel switch can be used to turn on (and turn off) 
the contactors in all the power controllers in the machine. I would guess 
(without seeing you system) that the 3 pin socket on the CPU box is wired 
so that you just plug the power control bus cable in there and it will 
connect the switch contacts between ground and ground-for-on.

-tony