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

[Tore Sinding Bekkedal]

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 :)