PDP-8 /e/f/m memory

[Don]

Chuck Guzis wrote:
> On 8/14/2006 at 9:35 PM Don wrote:
> 
>> Unless you can make a battery application last A VERY LONG TIME
>> (not just a LONG time since those are the cases where you get
>> screwed because you have forgotten about the battery that
>> you replaced 1 or 2 years ago... *but*, if you replaced it
>> *10* years ago you don't mind -- as much -- when it dies
>> after that long of a service life) it will frustrate users.
> 
> Well, consider the Dallas battery-inside-the-chip clock-calendar circuits.
> 7 years sounded like forever--and it may well have been to the original
> owners of the equipment.  But now, they're starting to fade and finding
> replacements can be a bit of a hassle.  In 20 years, it'll probably be
> nearly impossible.

Exactly!  That's why you don't use a proprietary battery!
(and, a battery *inside* a chip is the most proprietary
form I know  :> ).

I deliberately redesigned the clock/bbsram's in my SPARCs to
accept an external battery pack -- so I can just swap the
batteries when the time comes (instead of having to hunt down
replacement parts with stale batteries!)

> For this particular application, any keep-alive power source needs only to
> last as long the longest power-off interval.  If, as Don, says a supercap

But how often do you power up your '8?  I'm sure many folks
have them sit in a corner and only use them intermittently.
E.g., my Voyager hasn't seen power applied in 6 mos or more  :-(

OTOH, if you *know* that those AA cells will keep it
running for years AND you just discipline yourself to
put a new set in as a regular maintenance issue (just
like servicing your smoke detector, PDA, etc.), then
you never find yourself worrying if you've not powered
it up often enough to recharge a supercap!

> will last several months, that should be good enough--and it's a permanent
> solution.  Heck, I wonder if you could trickle-charge the battery with
> ambient light via a solar cell.    That should extend the keep-alive period
> some.
> 
> I just don't like chemical reactions in my equipment--it's sort of like
> bird's nest soup.  It might taste good, but I don't like the idea of eating
> bird spit..

I haven't found a "chemical mess" in a piece of equipment
since some paperclad eveready D cells leaked in a flashlight
a few decades ago.  I last replaced the batteries in the
SPARCs 3 years ago and my Compaq portable 3 was probably even
longer than that (it is a more "involved" effort to disassemble
than the SPARCs so it doesn't happen as often -- despite the
fact that the consequences of a BBSRAM data loss in the compaq
are potentially more severe than in the SPARCs).