[personal] Re: Computer (museum) registers [was RE: Modules for LINC-8]

[Mike Hatch]

Tore, hi..

A great story, mirrors to an extent my experience with the UK goverments 
PDP-7 #41, as a young engineer given the manuals and told "work it out", 
fell in love with computers especially the 7. We built an interface between 
the 7 and a PDP-11, a bit like Max Burnett's hybrid.

It's a pity your 7 is not working better, but at least it still "lives", and 
has not been sent to the dump, don't do that please.

Best regards,
Mike Hatch

Web - www.soemtron.org
Email - mike at soemtron.org

Looking for a PDP-7 (some hope!)


----- Original Message ----- 
From: "Tore Sinding Bekkedal" <toresbe at ifi.uio.no>
To: "General Discussion: On-Topic and Off-Topic Posts" 
<cctalk at classiccmp.org>
Sent: Tuesday, February 03, 2009 10:48 PM
Subject: [personal] Re: Computer (museum) registers [was RE: Modules for 
LINC-8]


>
> On 3. feb.. 2009, at 20.51, Pontus wrote:
>
>> Rich Alderson wrote:
>>> Where is there a register of PDP-7 systems?  I know of only two, 
>>> personally.
>>>
>> Its been on this list before, here is the link:
>>
>> http://www.soemtron.org/pdp7.html
>>
>> Lets hope the two you know about are not on the list :)
>
> That PDP-7 is in Oslo. However, it is not in operable condition.
>
> Long story follows:
>
> The PDP-7 was my first real retrocomputing project. In retrospect, it  was 
> probably a massively bad call for a first project! Before I began,  I 
> didn't even know what a capacitor was. The machine was in the  university 
> library in which I was hanging out. It was a better use of  time than 
> attending class. Although my grades suffered, I never did  really have 
> trouble justifying skipping a school FrontPage class for  deciphering the 
> inner workings of a computer. :-)
>
> Before I touched anything, I took the maintenance and user manuals  with 
> me to a week-long holiday at my family's summerhouse, and I still 
> remember the deep effect the F-77A service manual had on me. (now  there's 
> a sentence you don't hear every day...) I'd been a computer  geek before 
> this, but I had never quite understood computers past the  level of "I 
> input some mnemonics, and then magic happens inside the  chip".
>
> A few years later, I saw a video of Steve Wozniak explaining how he  "fell 
> in love" with the PDP-8 as he read the manual. He exactly  described my 
> own feelings. Although I had idly programmed computers  since I was 12 
> (Well, since age 9 if you count BASIC - but one  doesn't, does one...) 
> this was the first computer I felt I could  *understand*, and it made a 
> lasting impression on me and gave me a  lasting fascination with 
> retrocomputing as a way of "understanding"  computers and computer 
> engineering in a way that is simply not  possible with the vastly more 
> capable but yet somehow less interesting  modern systems.
>
> The PDP-7 documentation was "describing a world" which was immensely 
> fascinating to me.
>
> Anyway, back to the machine itself. Having read up on it, and  consulted 
> with electronics engineers (funny how those seem to be  abundant in a 
> CS/EE building, huh...) and also this mailing list, I  found that the best 
> course of action was to reform the capacitors in  the PSUs, and then test 
> the PSUs under a dummy load. The capacitors  all held a charge 
> marvelously, and were surprisingly close to their  labelled capacitance. 
> The PSUs were all within spec - not bad for a  system that hadn't seen 
> power since 1977!
>
> When initially powered up the CPU was completely dead. I managed to 
> locate a few problems with individual components and swapping the  boards 
> for working ones. (There was a cache of spare flip-chips - and  I refused 
> to allow a PDP-7 to become my first soldering job!). One of  my first 
> repairs, and the one that really got the system going, was  swapping out a 
> B204 -- IIRC, the faulty board had an off-value  resistor -- in the main 
> timing chain.
>
> By the time I was "done", the CPU was able to fetch, decode, and  execute 
> arithmetic, conditional branch, and OPR instructions - and  those were 
> just the ones I tested. However, when I STARTed the CPU,  the system 
> looped at location 0. I quickly found out why: The physics  department 
> had, to deal with an increase in I/O load, created their  own Automatic 
> Priority Interrupt (The paper I read described it as "a  poor man's API" - 
> I think it was submitted to DECUS).
>
> The professor who used the machine is quite tall, over 2 meters, at 
>  least - and is described by many as "Norway's (largest/greatest) 
> scientist". One time in the 1970s, he and a colleague of comparable 
> stature were at a DECUS or DECworld or somesuch meeting. The 
> conferancier, when receiving them, asked - "Are all Norwegians this 
> tall!?". Immediately, his colleague replied - "No - we were the only  ones 
> who could fit on the plane.". :-)
>
> The PMAPI was built out of 74-series circuitry. Of course, when the 
> system was decommissioned only a few years later, 74-series logic was 
> both bloody expensive and general-purpose, so those boards were  removed. 
> As a result of this, the CPU always loops on an active-low  IRQ from the 
> I/O rack.
>
> The absence of any I/O left me unable to test any of the other  peripheral 
> devices. The paper tape reader would start when asked to by  the CPU (The 
> binary load feature necessitated some direct glue between  the controller 
> and the CPU), The Teletype would transmit correct codes  as read by the 
> I/O rack status lamps. The TTY itself (a KSR33) had a  missing codebar 
> reset bail, and eventually the H-bar broke (wow, it's  been 4 years and I 
> still remember the name of the damned parts. The  Teletype manual was also 
> a fascinating read.)
>
> The core memory could store and recall worst-case noise patterns  entered 
> into the system by a program I wrote which I stepped through  while 
> holding in "CONTINUE".
>
> Considering how inexperienced and unknowledgeable I was, I'm damned  glad 
> I never managed to make anything catch fire, and as a bonus, I  think I 
> really got quite far all things considered.
>
> The wall-like learning curve was very interesting to climb and I'm a 
> happier person for it.
>
> Regards,
> -Tore :)
>
>
>