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Hybrid Lab in Hamerschlag Hall

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This page continues the documentation of the computers at Carnegie-Mellon University  in the 1960's. You can click on the links above to learn about some of the other computers.

Hi all,

Anyone remember the big analog computer in the basement of HH?


1969 Mark DiVecchio - Carnegie-Mellon University
Photo taken by either Pat Stakem or Chris Hausler. The arm is either Ray Carson or Ron Herold (now WD4IAD).

In a letter that I received from Dean Williams in 1966, he described "a special computer laboratory with a G-15D digital computer, TR-10 analog computers and a DDA-20 digital differential analyzer".

Which one was this one? It turns out - none of the above - read more below....

Here is a link to a TR-10 web site. Is it the computer we had?
It was made by a company named Electronic Associates Incorporated.
Subject:         RE: Your Arm
Date:            Sat, 27 Dec 2008 20:29:54 -0500
From:            "Ron Herold" <>


Looking at the physique of the arm in the picture you refer to - I don't think its mine.  It is in much too fine a physical shape.  I am in physical shape... Just remember Round is a shape.

The site is full of pix and memories.  Glad you have them down and visualized - as our memories fade.  College is so far in the past.
Best wishes to you and Sally for the holidays and coming year. Thanks for keeping in touch and find time to come visit.

Date:            Wed, 14 Jan 2009 10:00:01 -0800
Subject:         Analog computer photo
From:            Al Kossow <>

Hi, I hadn't stopped by the Athena site for a while, and came upon the photo in front of an analog computer "1969 Mark DiVecchio - Carnegie-Mellon University". It is a GPS unit. Very distinctive looking, and is similar in appearance to the unit we have at the Computer History Museum.

Note the company logos match..

I'm not sure of the exact model, guessing one of their last units from the 60's. If someone comes across documentation from GPS, please let me know since we have almost nothing in the archives on the company.

Hi All,

The Hybrid Lab was actually half a flight down from the parking lot and had a PDP-9 interfaced to an EAI 680 analog machine.  Apparently the 9 could turn the pots on the 680 which were designed to be turned by people.  Since it could turn them much more frequently than real people it was wearing them out :-)  The 680 was plug board programmed.  The 9 had DECTAPES and I still have one (the one "micro tape" I had originally purchased from the CIT bookstore for the PDP-8 but then could never use on it, as described on the web site) with PDP-9 FOCAL on it as well as some FOCAL sources I wrote.  Every time I would use the machine I would finish by having PIP print a directory of what was then on the tape to the 9's console.  The last printed directory is still folded up with the tape. Looking at this print out the tape also has a copy of those two "music" paper tapes I had gotten from DECUS for the PDP-9 as well as some other stuff.

Chris Hausler
I vaguely remember a "hybrid computer", with an analog computer connected to a digital computer.  On the analog computer, differential equations could be solved by including resistors, capacitors and inductors that were "analogous" to the problem you wanted to solve.  I seem to remember the analog computer connected to a DEC PDP 9.  I never actually used the system, so I don't know exactly how it worked.  Just like I remember seeing the Athena, but never actually used it.  I do remember a BIC pen in the Athena's plotter.  Really classy, I thought.

Bob McFarland
There is a photo of me in front that analog computer on the ASDG web page. I remember it as being on the ground floor of Hamerschlag also.

As I recall, I never got it to do anything intelligent.

Hi Mark,

That was the "other" analog computer which occupied the Hybrid lab for a while, not the 680 which was much smaller (actually smaller than the PDP-9 although not by much).  I know nothing further about it.  I did have a 680 manual (about the size of a phone book) but threw it out about 5 years ago (there is a limit to how much crap you can stuff into a small bag.  I keep testing this limit but every so often have to make room for new crap ;-)

Chris Hausler
Dave Vavra was the sys programmer for the hybrid lab - when we hear from him, it should clear up some of the questions.

The analog computer that I played with didn't have any digital computer interface.   May have been an earlier one.  We had some simple analog computers in high school so I actually knew how to program one.  I remember Frank Caimi and I doing some stuff on it to solve some lab problems.

Hi All,

Again, the machine in the photo is not the EAI 680.  See <> for an advertising brochure showing pictures.  It is advertised as a hybrid computer in that it has digital interfaces specifically designed for an external digital computer and refers to being ideal for new university "hybrid" programs.  Sound familiar.  As you can see from the photos in this document the 680 was programed on a plug board.  This allowed multiple users to wire up their experiment and then take turns running it with other users without having to un-wire between experiments.  The unit in the web page photo is not plug board programmable.

Also googling around I came across a second analog computer web site.  Going to its listings for EAI it shows a number of "PACE" models and although not exactly like the photo on the web site, damn close.  So the machine in the web site could be an EAI, just not the 680.  The reference is <>  If you go to the home page for this museum it lists many analog computer manufactures including Heath and analog computers tend to all have a similar appearance, panels with lots of places to plug patch cords.  There are a number of other sites with even better photos of the 680 than the above brochure, this all from typing "EAI 680" into Google.

Weirdly, the business I just retired from, automated railroad freight "hump" yards was done with room sized analog computers from the early 50's up well into the 60's.  To my knowledge the first digital computer yards were done in the late 60's and I did my first one in 1973, but after a couple more got into mass transit and didn't return to the hump yard business until the early 90's.  However we were still replacing old vacuum tube analog computers at times (as well as two of those digital systems I did in the early 70's, digital technology doesn't seem to last as long ;-)  I replaced my last large vacuum tube analog computer in Indianapolis in 2004 and there is still a similar one running in Buffalo, NY.  This was to be the next one replaced and parts were already being shipped to the site but last spring with the economy, the railroad decided to not only terminate the upgrade project but to shut down and abandon the yard.  It is possible, however, that cooler heads have prevailed.  It was believed that if they ever turned off the analog computer for any period of time, it would never run again.  Further, it is still maintained by an old guy who has been with it for most of its existence and if they shut it down he would retire and any knowledge of it would be lost.  At least as of a month or so ago, I heard the filaments are still lit :-)

Chris Hausler
I recall that we had the small desktop sized PACE TR-10 in room 55A. I never used it.

The larger machine was thought by a person from the Computer History Museum to be a unit made by General Precision Systems (GPS). He noted that the logo on the nameplate at the top of the unit looks a lot like the unit that they show on their web site:

But I don't have any hints about the model number of the unit at CMU.

Was this older unit replaced by the hybrid computer?

Hi Mark,

I believe the older analog unit came after the Hybrid Lab (EAI 680, PDP-9) was set up (unless it was hiding somewhere else around the campus and then was moved into the Hybrid Lab).  I don't believe it was there when I first started using the PDP-9.  I had assumed someone had cast it off and the EE department snagged it (Williams again?).  What with using the PDP-9 I got interested in the EAI 680, got that manual I recently threw out and sort of did a quickie do it yourself intro to analog computing.  With this, I drew up a very simple "toy" circuit to wire as an introductory example but there were no plug boards for the 680 available to me, they were all in use by more "serious" users so I never even got to try out my circuit.  Thus my career as an analog computer programmer was over before it began ;-)

Although I had come across that old ads site before I hadn't spent a lot of time with it.  I see the tape drives on the G-20 were made by Potter.  There's a nice brochure about the Univac 1004 III showing the Uniservo VI tape drive.  I remember seeing one of those Minivac 601's for sale in a local electronics store.

Chris Hausler

Hybrid Computation Laboratory - User Manual - Sep 1968

In March of 2010, Pat Stakem sent me a several inches thick manual from the Hybrid Lab. I'm going to scan it and make it available here. After I scan the manual, I will be sending it on to its final home.


Here are the sections (I will add links to the PDF file for each section as I scan them) :

Introduction (13 pages)
System Description (7 pages)
PDP-9 Operational Characteristics (7 pages)
Keyboard Monitor - An Operating Guide for the PDP-9 Monitor System (September 1968) (24 pages)
Linkage - PDP-9 Facilities (25 pages)
Operation - EAI 680 Operational Characteristics (12 pages)
Function Generator - A Package of Utility Programs for Function Generation (11  pages)
HYSAT - Hybrid Static Analog Test Program (23 pages)
Linkage - Hybrid Facilities (89 pages)
BLOCK CSMP-9 - A Block Oriented Continuous System Modeling Program for the PDP-9 (33 pages)
GPS - Operating Manual for the GPS Analog Computer (55 pages)

Pat sent me the scan of this set of Jan 1969 updates to some of the above documents :
PDP-9 Keyboard Monitor (KM9) Operating Guide and Other Document Updates (124 pages)

In Aug of 2013, Pat Stakem found this document from the CMU Comp Center and sent it to me for this web page:

1971 For Sale - EAI-680 & DEC-PDP9 Hybrid Computer  -  For Sale: EAI-680/DEC PDP9 Hybrid Computer. Total Original Cost   $160,705.00

Date:            Thu, 29 Aug 2013 12:29:24 -0400 (GMT-04:00)
From:            "J. Chris Hausler" <>
Subject:         Re: For Sale : EAI-680/DEC PDP-9 (1971 $$)

Hi Mark,

Interesting!  A while back we were chatting with Al Crew, I thought he said that the Hybrid Computer system was still in place when he was there, several years after this notice of sale and possibly what I saw in the back of the computer room on my fall 1976 visit with Pat and Russ.  Of course, the whole hybrid computer thing was kind of a flash in the pan.  Digital technology combined with numeric methods rapidly overtook any advantage possessed by a hybrid system and was certainly a simpler solution.  So I guess I'm not surprised that it didn't sell.  I wonder what was eventually done with the system as I don't recall seeing it on my next visit in the early 80's (82 or 83 IIRC).

73, Chris Hausler

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