[Info-vax] Databases versus RMS

[Johnny Billquist]

On 2012-04-21 19:00, Paul Sture wrote:
> On Sat, 21 Apr 2012 13:34:12 +0200, Paul Sture wrote:
>
>> On Fri, 20 Apr 2012 10:20:38 -0700, abrsvc wrote:
>>
>>> In the days of the stepper motor, that was a problem.  In more modern
>>> drives, the actuator arm is driven by a coil so no real friction or
>>> contact points.  Usually it wouldbe in the "home" position as well  The
>>> biggest problem was the current required to start the unit spinning.
>>> With the drives spinning, the amount of power required to keep it that
>>> way is significantly less than to start it spinning.  This allowed the
>>> total power required to both spin and record the data to be much less
>>> in both capacity and endurance.
>>
>> There was a way to link up RM05 disks so that if you switched them all
>> on at once they would only spin up one at a time.  The power draw of the
>> four we had spinning up at the same time was enough to trip the circuit
>> breaker.
>
> A kind soul has sent me more details on those:
>
> --- start quote ---
>
> The power bus was common throughout all of the older and bigger boxes; a
> room full of RA or RF disks all spinning up together would blow breakers.
>
>
> From<http://users.bart.nl/users/pb0aia/vax/750faq.html#powerbus>
>
> "What are the DEC Power Bus connectors for?
>
> On the back of the power controller, there are two three-pin DEC Power
> Bus connectors, one labelled 'Normal' and the other labelled 'Delayed'.
> These are used to allow the front-panel keyswitch on the processor rack
> to control the mains supply to the other racks in the system. Pin 3 on
> each of the connectors is grounded. Pin 2 is an emergency shutdown: when
> grounded, it shuts the power system down. This is normally caused by a
> temperature sensor in either the processor rack or one of the other
> racks, or by the airflow sensor in the blower plenum signalling that the
> blower has failed. Pin 1 on each connector is a power-up request signal;
> when grounded by the front-panel keyswitch, it causes the power-
> controller relay to close, supplying mains power to the PSU. On the
> 'Delayed' connector, the power-up request output is delayed by about half
> a second; switching the other racks in your system from this output can
> prevent those embarrassing mains surges that trip the breaker on the
> mains ring and plunge the house into darkness...
> Normally, the power controllers in all the racks of an installation are
> daisy-chained together, so that the front-panel keyswitch controls power
> to the whole system."
>
> --- end quote ---

The power control bus as such is very old with DEC. More or less the 
same design can be found already in the PDP-8.
However, the original RA drives did not use this. In fact, the delay 
output on the power bus at the power distribution was fairly late. I 
think I only saw it for the first time in the mid-80s or so.

The RA drives have their own power sequence bus. I don't remember if 
earlier drives had something similar. However, for the RA disks, you 
have the SDI cable from the disk to the controller. One per disk.
Then you also had the daisy-chained power bus cable. One drive (the 
first) would have a terminator on the input connector. That would enable 
it to spin up (note - all drives had power on at the same time, it is 
the spin up which is the problem). Then, the power bus output of the 
first drive went to the power bus input of the second drive, and so it 
continued. Each drive would only spin up when the input signal indicated 
that it was allowed to spin up. The output spin up signal would only be 
enabled once the drive had spun up.

This also caused (and sometimes still do) problems when someone gave you 
a drive. The drive will not spin up unless you have the terminator in 
the input port (or a cable from another RA drive).

The drives I know have this power bus is the RA80, RA81, RA82 and RA60.

Drives after that were no longer sucking as much power on spin up, so I 
guess DEC decided to skip this feature, in combination with the newer 
power distributions, which instead used the power bus as described in 
the quote.

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol