[TUHS] PDP11/34 boot problems, here i am again :)

[Carl Lowenstein]

> From: asbesto <asbesto at freaknet.org>
> To: tuhs at tuhs.org
> Subject: [TUHS] PDP11/34 boot problems, here i am again :)
> 
> well, i think now we have a more complete knowledge of the REAL
> problem !
> 
> the M7891 board (128K x 18 bit MOS MEMORY MODULE) have the D2 red=20
> led light turned ON when turning on the CPU !
> 
> this means PARITY ERROR on this board :(
> 
> checking the board we found 2 problems:
> 
> 1) a 74LS175 chip named E15 on our schematic diagram, that seem
>    phisically BROKEN on an edge (there is a fessure on the plastic
>    DIP package). i can't change it now because i don't have a=20
>    soldering station here to do a nice job, but looking the chip i
>    think it may work...

While you probably _do_ have a real hardware problem with the broken
IC, I think that the red Parity Error light on first turn-on is normal
behavior.  The MOS memory comes up with random values, and about half
of them will have the wrong parity.  Some software routine must turn
off parity error detection and write a known data pattern (all 0's) to
each memory location.  I don't remember whether this is something that
is done by the boot ROM of an 11/34.

    carl
-- 
    carl lowenstein         marine physical lab     u.c. san diego
                                                 clowenst at ucsd.edu