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Time to rebulid the pdp8/L

Hi Tom,

In my experience ALL TTL chips can break down, not just the 7474 for example. The 7474 and 7440 are used in huge amounts in these machines. So statistically you will find more of these defective. But there are chips like early ti and some other brands which have more problems. I have bad experience with Sprague chips as well... The quality of later generation TTL chips is way better. A SN7400 from 1969 can be questionable, but a SN7400 from 1980 will be very stable in my experience. I have had brand new 'old stock' 7440 chips from the seventies which I used to restore my PDP8/L. These ran for about 15 minutes and broke down again. No external cause, I replaced it with a newer model and the machine still runs fine.

In my experience the amount of broken TTL chips depends hugely on the quality of storage. Moisture and freezing cold seems to be a deadly combination.

Regards, Roland
 

Starring the Computer - DEC PDP-8/L

Computers in movies and television shows
www.starringthecomputer.com www.starringthecomputer.com

Somehow a strange stack of computers. PDP8/L on top that is a PDP11/05 and on top of that a 11/35?
Nice machines, but in that time you would not find suck a stack of only CPU's. It was more common
to have one or multiple racks for just one CPU... And next to it seems to be a Data General Nova.
To me this seems to be quite unlikely back in those days... But hey, I'm from 1977 so I was never there.
 
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Roland Huisman said:

Starring the Computer - DEC PDP-8/L

Computers in movies and television shows
www.starringthecomputer.com www.starringthecomputer.com

Somehow a strange stack of computers. PDP8/L on top that is a PDP11/05 and on top of that a 11/35?
Nice machines, but in that time you would not find suck a stack of only CPU's. It was more common
to have one or multiple racks for just one CPU... And next to it seems to be a Data General Nova.
To me this seems to be quite unlikely back in those days... But hey, I'm from 1977 so I was never there.
Click to expand...
I was there at the time, but never saw a PDP of any type or any other mini computer. At the time I was obsessed with "real computers", i.e. CDC CYBER mainframes and never thought much of "toy computers" like the PDP-8. My opinion has changed dramatically. I will never own a real CDC CYBER mainframe (although I wrote a nice emulator), but I can own, maintain and run a PDP-8/e. :)
Oh well - I am older and wiser (my wife disagrees on the last bit). :)

Tom
 
Roland Huisman said:

Starring the Computer - DEC PDP-8/L

Computers in movies and television shows
www.starringthecomputer.com www.starringthecomputer.com

Somehow a strange stack of computers. PDP8/L on top that is a PDP11/05 and on top of that a 11/35?
Nice machines, but in that time you would not find suck a stack of only CPU's. It was more common
to have one or multiple racks for just one CPU... And next to it seems to be a Data General Nova.
To me this seems to be quite unlikely back in those days... But hey, I'm from 1977 so I was never there.
Click to expand...

To quote my wife when I make comments like that; "Its just a movie".

The prop guys that raided the RICM warehouse just wanted things that looked retro-cool. They were not really interested in technical accuracy.
 
Well, in the pdp8/L and I the 7440's make sense as they were usually used as large load drivers. All 8 inputs were tied together and the one "output" signal would be used to run a whole bunch of other chips. So having those fail makes sense.

Likewise for the 7400's, most of the ones I had that failed had the inputs tied together and were used as inverters that feed a lot of other chips.

The 7474's were probably just complex chips and there's a lot of them all things considered.

@m_thompson Sorry to hear the 8/L is down again, your documentation inspired me to get this guy working. I'd say yeah: Test all the M series boards starting with the M700, then the M111's and the flip flop ones, then the ones with the 7440's. A single downed board can drive the whole system wonky.

Then focus in on the schematics and start by getting the M700 based manual logic to work.
 
m_thompson said:
The prop guys that raided the RICM warehouse just wanted things that looked retro-cool. They were not really interested in technical accuracy.
Click to expand...
Hollywierd wanted 9 inch tape reels spinning to and fro and more blinking lights than you could shake a stick at. Absent those, anything purporting to be a computer would end up on the cutting room floor. The IMSAI 8080 in "Wargames", a for real functioning computer, was a notable exception and even it got upstaged by "WOPR", a stereotypical empty box with myriad blinking lights on the surface.

This from Todd Fischer's web site:

The WarGames IMSAI – The Official IMSAI Home Page

www.imsai.net www.imsai.net
 
m_thompson said:
Found a bad 7440 in the RICM PDP-8/L. Now Load Address works. Examine and Deposit don't. A project for Wednesday.
Click to expand...
Yeah, the 7440's are especially suspect. I think it's because they drive a lot of chips and can blow out. Get a tube of them for spares.

And read through both of my threads, I started with getting LA working, then EX/DP, then fixed memory bits, then went on to Run, and finally to IO. Learned a lot, hope it helps you as much as some of the sources helped me.
 
And on a related note I got my Science Fair Digital Logic Lab out to build myself a JK flip flop. Turns out the 7400 on that is screwed, so I need to replace IT as well.

These chips did not hold up well (it's a TI 7400)
 
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