pdp-8/e restoration the next step.

Jon Elson elson at pico-systems.com
Fri Aug 4 21:24:22 CDT 2017


On 08/04/2017 05:31 PM, Rod Smallwood via cctalk wrote:
> Hi Guys
>
>         Thanks for the info so far. Well I have moved on. 
> I now have the minimum configuration.
>
> Programmers console, M8300,M8310,M8320,M8330,M849 and 4k 
> of core memory.
>
> The M8320 is at the far end of the bus.
>
> Well much as I feared I cant write to memory using Load 
> Address 0000, set data to 0001, lift DEP.
>
> Then set address to 0000 again and press EXAM. I should 
> see the contents of MD with the selector switch set to MD. 
> I don't.
>
> I have checked the correct bus signals go low when the DEP 
> switch is lifted.
>
> This is so fundamental somebody must have come across it 
> before.
>
OK, try reading a few locations and see if you can find a 
non-zero one.  If so, go back and read it again.  If it 
still reads the same value, that is a GOOD sign.  it means 
the write-back after a read worked.  If the 2nd read comes 
up zero, it means the write-back failed.  This would tell 
you that the write function of the memory is bad, but the 
read function is OK.

If some portion of the memory CAN read/write, then that 
indicates that either some decode logic or some select 
drivers are not working.  If nothing works, then it could be 
a configuration issue (the plugged-in memory module is not 
set at address zero, for instance) .  But, eventually, you 
would have to trace it through the prints.  Fortunately, the 
PDP-8 is not a terribly complex CPU.

Jon


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