PDP 11/40 Unibus termination query...

Josh Dersch derschjo at mail.msu.edu
Tue May 19 23:55:26 CDT 2009

Guy Sotomayor wrote:
> 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).

Hmph; just got done configuring the boards for the MMU :).  Is there a 
good reference for how the jumpers should be configured for just the 
basic CPU set?  I have the engineering drawing sets (in TIFFs... anyone 
have these in PDF? Tiff is annoying :)), and it indicates the jumpers, 
and _some_ of them seem to indicate a "default" position, but not all, 
and I don't know whether these are really defaults or not.  I know the 
MMU and the Stack Limit options require jumper changes, did any other 
options require them?

> 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.

Just assembled one of the KM11s last night, now I just need to order a 
couple of unibus extenders so I can use it :).

> 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).
The M9302 does not appear to have jumpers, and it doesn't appear to be 
of the same vintage as the 11/40 (it has a "caution" label that mentions 
an 11/34 or 11/04...) so maybe it is incorrect for use in an 11/40.

> TTFN - Guy

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