DS12887 pcb substitute with battery

systems_glitch systems.glitch at gmail.com
Mon Nov 13 10:01:49 CST 2017

Good stuff! I recently designed a module to build new DS1287 and DS12887
modules from the bare DS1285 and DS12885 ICs:


Just did a small run of 100 boards with the GW-12887-1 part number (they of
course work with the DS1285, though the number would be misleading). It
uses a 1225 cell as did the original, though the life of the original was
supposedly boosted by the potting not allowing the cell to evaporate as
much electrolyte through its (presumably imperfect) seal. I've see another
replacement that uses the surface mount version of the DS12885, but that
module requires surface mount bits and solder-down IC legs, whereas my
module just gets glued on top of the IC, and the appropriate legs bent up
and soldered into the castellated holes.

The motivation for the board was, as with yours, height restrictions. I did
a similar repair board for MK48T02 and MK48T08 timekeeper NVRAMs as used on
Sun systems -- there's an SBus slot directly over the NVRAM on some Sun
machines, which prevents the "glue a battery on top" method from working.


On Sun, Nov 12, 2017 at 7:44 PM, Maciej W. Rozycki via cctech <
cctech at classiccmp.org> wrote:

> Hi Chuck,
>  It's taken me a little while to respond to this one, however certain
> matters do need time to develop.
> On Sun, 22 Jan 2017, Chuck Guzis wrote:
> > On 01/22/2017 01:49 PM, Maciej W. Rozycki wrote:
> >
> > > A problem with reworking is there is sometimes very little clearance
> > >  available, so any modification has to be made in a clever way or you
> > > risk a short-circuit.  For example in DECstation 5000 systems their
> > > DS1287 chip is located in the TURBOchannel option slot area with the
> > > top of the chip almost reaching the solder side of the PCB of any
> > > option plugged in there.
> >
> > Although there are several illustrations of the DS1287 rework where a
> > coin-cell holder was placed atop the chip, there's absolutely no reason
> > why that has to be so.  A couple of wire leads away to an off-board
> > battery holder, holding, say, an ER14250 half-AA.will work just fine and
> > keep the battery away from the PCB--or a couple of alkaline AA cells,
> > which should last a few years.  No reason to stick to lithium.
>  Yeah, putting the aesthetic of the solution aside, in the DECstation
> there's the practical issue of making sure the wires do not obstruct
> shuffling TURBOchannel option cards and also finding a place where the
> substitute battery does not disturb the airflow such as to cause a
> component somewhere to overheat -- the whole side of the enclosure is a
> grille.
> > You know--"use your wetware".
>  That always helps, as long as it's got enough input to process, in
> particular as to what resources are available.
>  Mine has got that now, thanks to this thread of discussion, where I
> learnt of the existence of this small coin cell holder, and also the other
> thread mentioning wire-wrap wire and its excellent properties for small
> circuit repair/modification works.  Naturally I picked up the green
> insulation color to match what DEC used to use, at least in their
> TURBOchannel equipment. :)
>  So here: <ftp://ftp.linux-mips.org/pub/linux/mips/people/macro/ds1287/>
> are a couple photos of the result, meeting both the clearance requirements
> and my subjective aesthetic criteria, and avoiding the issues I identified
> above.
>  Taking into account the rated DS1285's standby power consumption of 0.5µA
> and the 35mAh 2.5V discharge level capacity of the CR1220 cell it will
> guarantee keeping the chip powered for 8 years.  Which I think is enough
> given the ease to replace the cell now.
>  For comparison the 48mAh capacity of the BR1225 cell originally embedded
> in the chip allowed for 11 years of operation and the CR2032 cell people
> seem to commonly choose allows for 50 years at its 220mAh capacity (which
> I find a bit of an overkill).
>  Note that I have calculated approximate cell capacities based on
> discharge characteristic curves published by cell manufacturers, as the
> rated capacities published are calculated for the 2.0V discharge level
> whereas DS1285 is specified for 2.5V minimum battery voltage required for
> correct operation.
>  For the record, I have measured the clearance available and it's 0.1875"
> (3/16") or 4.76mm from the top of the RTC chip to the bottom of the PCB of
> a TURBOchannel option right above it.  The TURBOchannel mechanical spec
> allows for 0.075" (3/40") or 1.91mm underneath component or lead height
> (e.g. 24-plane HX+ options have memory chips there).  Which leaves 0.1125"
> (9/80") or 2.86mm of space only.
>  I may yet stick a piece of insulating tape on top of the RTC chip just to
> make sure no contact happens by chance, though I think it might be an
> overkill after all.  It's not like the cell is going to pop out by itself.
>   Maciej

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