VAX-11/750 EMC Memory Board LEDs

Johnny Billquist bqt at
Thu Jul 9 04:36:08 CDT 2009

David Comley <david_comley at> wrote:
> --- On Wed, 7/8/09, Ethan Dicks <ethan.dicks at> wrote:
>> > One possibility is an ECC fault.? ISTR that since the
>> > boards fire up
>> > with zero or random contents, most/all of the locations
>> > don't have the
>> > right check bits.? The board sets the checkbits all at
>> > once at
>> > power-on, possibly with some sort of startup/refresh
>> > counter, so it
>> > might take a noticeable time to strobe through the array.
> Yes, that seems reasonable. The puzzling thing is that on all the boards, the light stays on, just less brightly, after a few seconds. 

Sounds unlikely that they indicate ECC errors if the are on most of the 
time, even if dim. Access seems much more likely.

> Is it the memory board or the memory controller that sets the checkbits ?

It is the memory controller. The board itself is a pretty stupid array 
of memory cells. Even ECC isn't handled on the board, if I remember 
right, but is handled by the controller. The board just gives 39 data 
bits for every 32-bit word.

And at startup, every memory board is accessed in parallell by the 
controller to initialize the ECC bits. So they should all be glowing 
brightly at startup if it is an access indicator.
But in that case, I would expect them all to then turn off as no memory 
access will be going on after that, until the machine boots.

>> > dark?? Are your RAM chips socketed by chance
> No, they're soldered in.
> Each one has a switch that I believe allows you to take the board offline. I can take any board offline (which throws a red memory configuration error lamp on the memory controller) except for the board in the left-most slot, no matter how few or many of these things I put in there.

That makes sense. If you remove any board in the middle, you get a weird 
hole in the memory map, which is illegal. The board at the left end is 
the last board, so removing that one isn't a problem. You don't create a 
hole then, you just reduce the amount of memory you have.


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

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