ISO: PDP-11/40 LTC and Stack Limit options

Josh Dersch derschjo at
Tue Nov 22 18:40:56 CST 2016

On 11/22/16 4:00 PM, Josh Dersch wrote:

> On 11/22/16 9:27 AM, Noel Chiappa wrote:
>>      > From: Josh Dersch derschjo at
>>      > see if the same is true for other bus grants -- I can run the 
>> system
>>      > with no grant continuity card at all in slot 9 and everything 
>> works.
>> Well, the BG4-BG7 grants definitely _are_ run through the SPC slot 9 
>> (see
>> below) - at least, on a stock system. It's _possible_ that the software
>> you're loading doesn't use interrupts. (I have this vague memory 
>> that, unlike
>> the -11/34, the /40 doesn't complain if there's a non-continuous 
>> grant line.)
>> Or perhaps someone wired them across on that slot, to avoid 
>> knuckle-mashing
>> trying to put a G727 down there.
> Yes, I'd expect them to be run through the slot (though I expected the 
> NPG, too :)).  And there does seem to be continuity between the BG 
> pins on the CPU backplane and those on the DD11.
> If I remove the grant continuity card from slot 9, I can still boot 
> XXDP.  If I remove a grant continuity card from the DD11 (with NPG 
> still intact), I can't  -- it hangs as I'd expect with a hole in the 
> grant chain.
> No one's done anything cute like hard-wiring the grants in and there's 
> no evidence of any modifications.  All expected voltages are present 
> on the backplane pins in the right places.  I put a little contact 
> cleaner in the slot too, just in case.  I still can't get an SLU to 
> function in that slot, though the CSR addresses seem to respond and 
> the console/diagnostic PROM chugs along happily when I power the 
> machine up (though nothing appears on the serial line).
> Tonight I may try running the SLU on an extender board and verifying 
> that all the proper voltages are actually making it to the board. The 
> fact that the board appears to be responding but I'm getting nothing 
> over the serial line makes me think that maybe the -15V isn't present 
> for some reason...

Annnnd: mystery solved (?).  I had borrowed the SLU I was using in the 
11/40 from my 11/34.  This was an M7856 (DL11-W).  Thought I'd grab 
another SLU from my UNIBUS drawer and configure it up so I could put the 
7856 back in the 11/34, but all I had left were the older, more annoying 
to configure M7800s (DL11-D).  So I jumpered it up (max of 2400 baud 
with the crystal that's currently installed, blah) and installed it.  
Works fine in the DD11 backplane.

Moved it to slot 9 of the CPU backplane.  Still works.

So... maybe I just completely missed the memo on this, and maybe there's 
still something wrong with my backplane... is it possible that slot 9 of 
the 11/40's CPU backplane is wired *specifically* for an M7800?  I guess 
I need to spend some time tonight seeing what the differences are... but 
now I need to go attend to a cranky 2yo who just woke up from a nap...

- Josh

More information about the cctech mailing list