More PDP-11 questions

Henk Gooijen henk.gooijen at
Tue Feb 10 01:03:56 CST 2009

> From: toby at
> To: cctalk at
> Date: Mon, 9 Feb 2009 20:51:16 +0000
> Subject: More PDP-11 questions

> ... snip some text ... 
> 3. I have an 11/34A (in 5 1/4" box) which is experiencing a similar
> constant run light problem to the 11/05. The machine has CPU cards,
> terminators, console card and a MOS memory board in it. With both
> terminators in, run light stays on. If I halt it, the bus error light is
> illuminated.
> If I remove the end terminator and power on, the console is responsive
> and I can deposit values into memory and read them back.
> Any ideas?
> Many thanks,
> Toby


Hi Toby,


the problem in your 11/34 sounds like a non-continuous grant chain.

Especially when you remove the terminator at the end, and the

system does not hang any longer!

Start with a minimal system. So the DD11-PK (first backplane)

only contains the 2 CPU boards, the M9312, the console interface

board M7859, and the SLU M7856 for the console terminal. Remove

the memory module, and put the terminator M9302 in the 9th slot,

in positions A-B.

Put in *all* other slots that have position C and D free a G7273 card.

(I hope you have those, you need 5 of them).

(I assume that are the cards you have in your 11/34)


With the G7273's you made sure that NPR grant chain is intact.

When you power up the /34, RUN should stay off, or pressing CTRL

and HALT should make it go off.


If you reached this condition, you can remove the G7273 from the slot

where you want to install the memory board and install the memory.

Test again.


Then remove one by one, one G7273 and replace it with a G727A, aka

knocklebuster. At a point the RUN LED will no longer go off. That slot

has the NPR wire (pin CA1-CB1 connection) cut. That is a slot you

can use for a "DMA" device like the RL11 interface. If that slot is not

used, you must have an G7273 inserted there, or a G727A *and* re-

wire pin CA1 to CB1.

In the end, some slots will have G7273's and some will have G727A's.


Needless to say, that for preservation of the hardware, switch off the

power before swapping even a module that has no components like the

G7273. And do not switch the system on within one or two seconds

after switching off. A few seconds delay will not hurt!


- Henk, PA8PDP.


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