Reset NVRAM on DECstation 5000/240

Maciej W. Rozycki macro at
Tue Aug 1 13:37:37 CDT 2017

On Mon, 31 Jul 2017, Rob Jarratt wrote:

> >  It is.  Check the "DECstation/DECsystem 5000 Model 200 Series Maintenance
> > Guide" (EK-PM38C-MG-002), available online.
> > 
> I have that manual. Having checked again I realise I should have searched
> for "NVR" and not "NVRAM", then I would have found it!!

 Or "jumper" would do too, ;) avoiding DEC documentation peculiarities 
(they indeed call the Dallas chip "NVR", reserving "NVRAM" for the MS02-NV 
Prestoserve SRAM memory board).

> >  Technically it's wired to one of the 16 IOASIC external interrupt inputs,
> which is
> > otherwise unused.  Functionally it tells software whether the mainboard is
> used
> > in a 5000 or a 5900 system configuration.  I can dig out the exact details
> if
> > you're interested.
> That would be interesting, if there is documentation that isn't generally
> available it would be good to see it.

 Not everything has been properly documented, I've gathered information 
from various sources, including reverse engineering.  The bigmax IOASIC 
IRQ bit is I recall only mentioned in an Ultrix error log processing 
utility, though the jumper itself is I believe mentioned in one of the 
5900 manuals, available online.

 Over the years I've gathered available information in Linux kernel 
headers; for the KN03 mainboard specifically:


Interpreted along with:


and knowing that in the console you need to use the MIPS KSEG1 uncached 
address space you get:


which expands to:

0xa0000000 | (0x1f800000 + 1 * 0x00040000 + 0x110) => 0xbf840110

so to get at the interesting bits:

>>e -wx 0xbf840110
bf840110: 0x00008802

i.e. the bigmax bit is set as for the 5000/240 (jumper present), the NVR 
clear bit is clear (jumper absent), and the NRMOD bit is clear (jumper 
absent).  There is also a TURBOchannel slot #0 option interrupt pending 
here for some reason and the halt button is released.

> >  BTW I saw your 5000/240 web page and I'm glad that you have rescued all
> > that fine hardware.  Also I think you don't have to worry about the H7878
> PSU --
> > apparently none of the caps used in that model suffers from the dreaded
> > quaternary ammonium salt issue.
> Thanks. I will create a page describing the machine in the coming days, for
> now it will just be the blog posting. I do think there is a problem with the
> PSU though because it will be just sitting there in the console and power
> off, you can't turn it back on until you pull out the power cord. I am going
> to check the caps more closely.

 Always worth doing of course, although from its arrangement the H7878 
would be a pain do disassemble to reach some of its caps.  Also check for 
the PSU overheating condition, reported at bit #4 in the IOASIC interrupt 
status as examined above (with good status indicated with the bit clear).  
It may cause the PSU to eventually shut itself down.


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