VAX-11/750 EMC Memory Board LEDs

Ethan Dicks ethan.dicks at
Wed Jul 8 15:17:39 CDT 2009

On 7/8/09, Ian King <IanK at> wrote:
>  > Can anyone tell me what the yellow LED signifies on the EMC 1MB memory
>  > boards that can be used in the 11/750? The EMC part number of the
>  > boards is 240-011-900.
>  >
>  > The green LED appears to be for power but I'm curious about the exact
>  > meaning of the yellow LEDs since they start out bright, on powerup, and
>  > then dim quite dramatically after a few seconds, presumably after or
>  > during initialization of the boards.
> We have some of those in our 11/785s but I was never able to find any documentation.  I think your interpretation is likely correct.  When I was having problems getting one machine to boot, those LEDs stayed on.  -- Ian

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.

Another possibility is that they are a simple access light - I think
I've seen those on some 3rd party board - again, at power-on, things
aren't stable yet, so you get solid lights until the memory board
finishes its own startup.

Do the yellow lights seem to vary in brightness under different
loading conditions during normal operation or are they essentially
dark?  Are your RAM chips socketed by chance (probably not - the last
socketed memory I remember seeing was Qbus memory for PDP-11s and the
uVAX-I).  If so, you could pull one chip from one board, forcing an
ECC error and watch the behavior.

If you have a red LED, too, that might be a hard fault, so the yellow
could be soft fault or access.

It's been a long time since I've played with 3rd party memory on a
VAX, so forgive me if I'm tangling my own memories here.


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