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

Josh Dersch derschjo at
Wed Nov 23 01:49:44 CST 2016

On 11/22/16 5:13 PM, Guy Sotomayor Jr wrote:

>> On Nov 22, 2016, at 4:40 PM, Josh Dersch <derschjo at> wrote:
>> 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…
> I seem to recall something similar with one of the 11/40’s I was working on.  I believe that because the 11/40 CPU backplane is wired for an M787 that things don’t work quite right if you try and install an M7856 into the SPC slot in the backplane and try and use it and not the M787.  There is a way to jumper the CPU backplane so you can use na M7856 and not the M787.  I haven’t really investigated it in detail (and it’s been a long time since I messed around with the 11/40 at that level).
> TTFN - Guy
Interesting.  I suppose the LTC on the M7856 could also be disabled... 
I'll have to try that and see if it works.  The DL11-W manual doesn't 
seem to cover any special installation procedures for the 11/40, and the 
11/40 System Manual describes Slot 9 as a "SMALL PERIPHERAL CONTROLLER 
(USUALLY DL11)" slot without mentioning the lack of NPG and the LTC 
oddity.  Weird.

I now have the CPU passing diagnostics with flying colors (there was a 
problem with the IR Decode logic causing incorrect reserved instruction 
traps), and I can boot RT-11 from the SCSI controller, yay.  Now to 
track down the EIS and Stack Limit options...

- Josh

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