PDP 11/40 Unibus termination query...

Guy Sotomayor ggs at shiresoft.com
Tue May 19 22:57:57 CDT 2009

On May 19, 2009, at 8:25 PM, Josh Dersch wrote:

> Hey all --
> I'm one step closer to bringing my 11/40 back to life -- the front  
> panel is now responding and I can examine and deposit memory.
> But the machine is only responsive without the Unibus terminator (an  
> M9302) installed.  If it's installed, the front panel is basically  
> hung -- toggling "Start" causes a brief flurry of activity, but  
> that's the only thing that causes any response.
> Without the terminator installed, the front panel more or less  
> works, I can examine and deposit memory, load the address register,  
> etc... but I can't get any toggled in code to run, obviously -- it  
> traps to the bus error vector at 00004.
> (There's also an odd issue, which I doubt is related, but Examining/ 
> Depositing does not correctly increment the address -- starting from  
> 0, it's "0, 2, 6, 12, 16, 22, 26..." and if I start at 1 it's "1, 3,  
> 7, 13, 17, 23, 27...")
> I currently have the CPU boards + MMU option installed in the  
> correct order with a SLU card in the SPC slot, and an M981  
> connecting to a 4-slot Unibus backplane with a single 64K MOS memory  
> card (M7891) in slot 2 -- all other slots have grant continuity  
> cards installed.  The Unibus terminator is installed in the last slot.

The first thing that I always do on an 11/40 is make absolutely sure  
that all of the jumpers on all of the CPU boards are set properly.   
DEC decided it wanted a lot of flexibility in 11/40 offerings.  The  
result is a lot of jumpers spread between all of the CPU boards.  If  
they are not *all* set properly you'll get strange behaviors.

If the boards are in an unknown condition, I would configure the CPU  
to *not* use the MMU until you have the basic CPU running and passing  
some basic tests before adding in the MMU back in (yea, I know it's a  
pain because you have to solder/cut jumpers to change the config).

Since the entire 11/40 CPU is microcoded (including the operation of  
the front panel) you might want to use the appropriate KM11s and  
single step through some of the micro code.  This can help pin-point  
the fault.

I also don't recall at this moment the M9302, but many of the  
terminator boards have jumpers for the different types of pull-ups for  
some of the processor signals.  If the pull-ups are incorrectly  
configured for the type of processor, the unibus can hang.  Remember  
the unibus needs to be terminated at both ends.  However, some signals  
are only to be terminated at one end (ie some CPUs terminate the  
signals themselves and others don't so the terminator at the end of  
the bus needs to be configured properly).

TTFN - Guy

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