PDP-11/44 problem - RUN stays on

Tony Duell ard at p850ug1.demon.co.uk
Wed Nov 15 17:32:41 CST 2006


[...]

> Turn on the system ... yep, a hang condition and the FAULT light
> on the RL02 goes ON immediately!  Removed the MS11-P again, and
> everything looks healthy!
> 
> It definitely looks like a defective MS11-P. The effect that it has
> on the system (especially the RL02's FAULT light) gives me hope to
> be able to repair that module one day!

No, I'm really guessing here, but to me that sounds as though that 
defective memory board is asserting one or other of ACLO and DCLO (you 
should check this with a logic probe or whatever). 

Now why should a memeory board assert one of those signals? Well, I am 
digging into the depths of my brain, but I seem to rememebr at least one 
DEC memory board for the 11/44 did some quite complex initialisation at 
power-on (basically it wrote to every word of the memory array to put 
valid parity or ECC (I forget which it used) bits in place). During that 
initialisation, it prevented the processor from starting by (you guessed 
it), asserting one of ACLO or DCLO. In which case, if there's a fault in 
that initialisation logic, it could permanently assert said signal.

As I said, it's a guess. But it might give you a place to start looking.

-tony



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