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

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

[asbesto]

well, 

some months ago i asked for help booting a pdp11/34 here at
freaknet medialab :)

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 
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 
   soldering station here to do a nice job, but looking the chip i
   think it may work...

2) a 5 Kohm 4-resistor bridge named R22 on our diagram, that is phisically
   BROKEN. this bridge give +5V on signals named DATA OC L, CSR OC L, OC L
   and X ADD OC L. i changed it with a 4.7Kohm 4-resistor bridge, without
   any result. (maybe the 74ls175 is really broken :)

an other ipothesys may be: some electrolitic capacitor in short circuit.
we had this problem on a decwriter III ! :)

the real problem is: if we aren't able to repair the M7891 board, where
can we find a "new" one ???

can somebody help us ? it's a shame for us to have our pdp11/34 offline :((

any help is appreciated ! *:)

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

[asbesto]

Il Tue, Apr 15, 2003 at 12:42:24PM -0700, Carl Lowenstein rigurgitava:

> 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.

hmmm
but running any code on the pdp11/34 (specifically, some boot 
code) will also result in the red light turn off and on again :(

-- 
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[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

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

[asbesto]

Il Tue, Apr 15, 2003 at 08:37:20AM +0200, Gooijen H rigurgitava:

> > 1) a 74LS175 chip named E15 on our schematic diagram, that seem
> >    phisically BROKEN on an edge (there is a fessure on the plastic

> I never remove IC's in *one* go. The chances that you damage the
> multi-layer board beyond repair are real. This is what I do.
> I use a dremel tool (small motor with sharp rotating blade) to
> cut the pins of the IC one by one.

yep. me too. i prefer this way. :)))

pray for me ! :)))

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

[Gooijen H]

"asbesto" wrote:
> 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 
>    soldering station here to do a nice job, but looking the chip i
>    think it may work...

I never remove IC's in *one* go. The chances that you damage the
multi-layer board beyond repair are real. This is what I do.
I use a dremel tool (small motor with sharp rotating blade) to
cut the pins of the IC one by one.
Don't use a cutting tool to cut the pins because the side force
might push the remainder of the pin through the solder joint
thus damaging the board.
After you have cut all the pins the IC just falls of the board.
Now, you can use a fine hot soldering iron and remove the pins
one by one use a pair of squeezers.

  good luck with your repairs,

- Henk.