Even more PDP-8/A weirdness

Jon Elson elson at pico-systems.com
Sun Sep 13 11:02:48 CDT 2015

On 09/13/2015 02:52 AM, Henk Gooijen wrote:
> -----Oorspronkelijk bericht----- From: Charles
> Sent: Sunday, September 13, 2015 12:36 AM
> To: cctalk digest
> Subject: Re: Even more PDP-8/A weirdness (was: Re: PDP-8 
> diagnostics SR settings?)
> As previously posted, reseating the boot ROMs in their 
> sockets cured the
> corrupted boot loader issue. Got lucky there :)
> As before, the AJRLHA diagnostic (Seek/Fctn) still faults 
> and prints errors,
> but only at the start of testing the drive. Thereafter 
> that drive shows no
> errors. Then the program will switch to the other drive 
> and the same problem
> occurs. Specifically (edited and reformatted for clarity, 
> with results
> interpreted from the RL02 user manual):
> [... snip ...]
> Any help making sense of this mishmash of errors would be 
> greatly
> appreciated :)
I'm pretty far away from competence on PDP-8s anymore, but 
the symptoms sound like maybe the drive faults on LONG 
seeks, but as long as the seeks are short, it works OK.  
There might be a one-shot in the controller that allows so 
many ms for a seek to complete, and due to aging capacitors, 
the delay is now too short.  But, that's a totally wild 
guess, there could be troubles in the drive seek electronics 
that only occur on longer seeks.


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