PDP 11/24 - A Step Backwards

Chris Zach cz at alembic.crystel.com
Mon Mar 28 14:57:00 CDT 2022

> I don't think the CPU is working at all. The reason being that there is
> absolutely no LED activity. Including an LED that is supposed to indicate a
> clock. Having hopefully eliminated all the power voltages it left me
> wondering if there was a fault on the CPU or in the PSU. Having had activity
> on those LEDs recently it seems most likely that it will be the PSU,
> particularly as *something* in there blew up. The only signal that I can
> identify that seems likely to have this kind of effect is LTC, but I don't
> know enough about LTC to know if its absence could cause the CPU board not
> to work at all, although I see above that you think it unlikely. I suppose
> the fault could be something I can't see on the CPU board, particularly as
> there do seem to be some quite large spikes, otherwise I am not sure if
> there is anything else from the PSU that could prevent the CPU getting
> going.

I'm on a nice long train trip right now but I recently got my 11/24 
running again. One thing that baffled me was it would not do anything on 
the serial port. No ODT, no nothing.

Turns out you really need to make sure the slots are filled properly. 
The CPU top, then the memory map, then for the next 4 boards one has to 
be either a properly configured MS11-PL (the 128kw board) or the memory 
boards specific to that type of 11/24. Or you have to put G7273's in the 
CD slots.

Next you need proper devices or G7273's in the next two slots and a 
proper terminator in the left sockets of the last slots and a G7273 in 
the center slots. Only then will ODT work.

Another oddity is that the 5.25 inch box has +5 and +12 I think and the 
10.5 has +5 and +15. There are different memory boards that work in one 
and not the other, or both depending on jumper settings that have to be 
right. Unibus drives me bonkers sometimes with the number of different 
voltages requires (+5, +12, +15, +20, -15, etc....) It doesn't help that 
the +15 and +12 are on the same pins.

Plus it's possible someone screwed with some switches, make sure they 
are set properly (ie: default is a good start).

If you're still stuck next week drop me a line and I'll fire up my 11/24 
and see if I can replicate your failure.

>> The first will tell you that i) the CPU is basically functional, executing
> micro-
>> instructions, etc; ii) that the bus is basically functioning (for
> master-slave
>> cycles; DMA and interrupts will remain to be checked out); iii) that the
>> console port is working. (Yes, on the KDF11-U, the console is on an
> internal
>> bus, and so in theory a machine could have the ODT 'front panel'
>> working, _and_ still have a problem with the bus, but depending on the
>> exact details of said problem, maybe not.)
>> So, hook up a console, set the machine to 'halt', and power on. Is console
>> ODT working? If so, congrats, you win, go to stage ii) below.
> I had a console attached. There is nothing on the console. When I first got
> the machine I did get output on the console. But that was before the PSU
> first failed on me, which is quite a few years ago now.
>> If not, you have a reduced area in which you have to investigate - and
> you'll
>> need a 'scope of some kind to make any progress. (If you don't have one,
>> you're SOL. Get one.). In order i) is the CPU's internal clock (and thus,
>> probably the microcode) running? (At this point you will need to consult
> the
>> "PDP-11/24 System Technical Manual", EK-11024-TM.) If so, is it trying to
> talk
>> to the console's registers? (See Section 4.6 of the TM, "Internal Address
>> Decode".) If so, is the UART working properly? (4.7 of the TM, "Serial
> Line
>> Units".)
>> If so, console ODT is running, you're now at stage ii): you can see if the
>> will run. Deposit a 0777 ('BR .') in a likely location (I usually use
>> 0100 or 01000); read it back to make sure the write succeeded. (If not,
> likely
>> either the UNIBUS or the main memory has a problem.) Start the machine;
>> the 'Run' light should come on - if you're lucky!
>> Depending on which bin you wound up in, further assistance s available.
> :-)
>> 	  Noel

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