Burroughs B7800 Mainframe

abstract: Burroughs large systems memorabilia, B7800 parts
author: Ralph Klimek
date: Mon Apr  7 18:05:00 EST 2008
copyright:  this article is copyleft, no rights are reserved, logos , trade marks, however, clearly still belong to their respective owners

My involvement with Burroughs and the mainframe worlds was from late 1985 through to 1990 when by employer ended its usage of large mainframe systems and replaced it with various minicomputers and migrated the legacy databases. There I labored under the steady and true gaze of the great Mr Bruce A. Seaman one of natures gentlemen and the greatest engineering manager that I have had the privilege to work for.

In that time I performed engineering maintenance functions on the mainframe and its peripherals. Sadly very little of it remains, no documents from that time have survived and only a few relics and only one museum piece remains from that era.  This was well before the days of the web, near infinite disk storage and digital cameras , so all thats left are rapidly fading memories.

I have come to believe that Monash University may have been the only organization on the planet that did total hardware maintenance of it Burroughs Large Systems completely in house.  I wonder how true this actually is ? Were there others ?

Memoirs of a mainframe engineer,
my time as a Burroughs B7800  "field" engineer at Monash University

This piece of the right hand portion of the front panel from a Burroughs B7800 CPM (central processing module) now decorates my garage door where it awes my enemies and impresses my friends.
My souvenier of the Burroughs B7800 CPM. In this design, every register and flip flop was brought out to the front panel. Each LED had about 16 different meanings depending on the switch settings. The display was allways on and looked magnificent. You would walk amongst rows of these panels, it was like the imagined space ship main battle station of the Death Star.  It was a great crowd pleaser at open days and I am sure that this display made the job of Burroughs salesmen so much easier. As an engineering tool, it was usefull but not essential. We had a B800 mini computer whose only job was to read the front panel and tell us that led 27, row18, panel 2 was off. We would walk around and sure enough the led was in the wrong state. Very powerful, wise and all knowing it made us engineers feel!

This is an image of the late Dr. Clifford Bellamy  Computer Centre Director Monash University

This was our Director and my Boss's  Boss.  Dr. Bellamy  was the driving force behind many totally remarkable and courageous initiatives that put Monash University in an very enviable global position in the 1980s.  He directed the existence of projects that were completely from "left field"  and would have left other erudite institutions gasping in disbelief.  It was his direction and faith in the capability and motivation of his staff that we were able to do what few institutions were able to do successfully, which in my sphere was the complete maintenance of a very large third generation mainframe computer in house .   It was not just the Burroughs  either.  The engineering team,  as lead by my Boss, Mr. Bruce Seaman ,  also did complete inhouse maintenance of a large fleet of VAX780 "super"-mini computers, a large fleet of Pyramid 90X unix minicomputers and all attending peripherals.  Dr Bellamy also conceived of the the "Monet" local area networking system which the image shows him displaying.  

 "Monet"  was a fantastic mechanism for the transport of serial terminal lines. In the era of the dumb green screen terminal or which Monash had thousands  distributed throughout the large  clayton campus,  it was not physically possible to distribute RS232 lines throught our square kilometer campus.  Monet was a networked terminal line concentrator system that had local nodes that gathered terminal data lines and put the data, without loss, onto  custom 2M/bit/s balanced data CD/MA  buses that spanned the campus. This was turned into a successfull commercial products by RACAL. Monet also led to the creation , in house of a complex   UNIBUS processor card that plugged into a VAX780 that demultiplexed hundreds of serial lines  directly into the VAX. Custom VMS drivers were also written to do this.

One of the totally remarkable projects he started was the creation , in house, of expanded memory cards for the VAX780. We took standard memory cards that were fitted with 16k-x-1 dynamic ram chips put in  256k-x-1 chips.  This also required considerable modifications to the memory cards and rewiring of the VAX780 backplane. It was a change that when started , could not be backed out. It also had the result that now our VAXes could NO LONGER be maintained by DEC. Could you imagine  contemplating something like this is this day and age.  In those days a 1 megabyte memory card would have sold for $30000.

The result of his unique in house initiatives was that Monash had a fleet of computing resources that was at least one order of magnitude larger than our competing tertiary institutions, yet we had comparable budgets.

In my pantheon of personal heroes, Dr Bellamy occupies a special place.

The Burrough's B-Line  an in house journal from the early sixties. This is pioneering computing at its best

click me to read the entire edition

an earlier edition of the Burroughs B Line

The smaller left hand panel of the Burrough B7800 IOM, the Input Output Module was a standalone processor that handled all interactions with the peripherals; disk, tape, printers, TTYs

This is a head assembly of the head per track disk right (see images in The B-line) The ruler is a 15 inchOne disk from a Burroughs "DISK" controller as distinct from "PACK" The "DISK" was this massive one head per track monster. Total capacity was only 5Megabytes but was damned fast because there was no seek delay. On this was implemented some of the industries very first effective virtual memory, as this monster was still cheaper than core or the new fangled solid state memory. 

b5500 core memory
The above image shows a plane of B5500 magnetic core memory, image courtesy of Mr Alan Thorne who actually used it ! See the irregular spacing of the rather large toroids, for this core plane was hand threaded. The higher density core plane, elsewhere was machine threaded and is more regular.
These core planes were made in Hong Kong, presumeably by the poor folks that had escaped from communist China and would do this highly detailed and tedious work for nothing more than a beating, a handfull of rice and a vague promise that things might get better. Click on this image to get the full resolution.

The console from a Burroughs Disk Pack Driver Controller. There were two redundant controllers in every B350 controller cabinet. The controller was a B700 minicomputer in its own right. The console could be inserted and removed while online. It was removed to discourage "ge-finger-poken".  I used these regularily to perform disk pack maintenance after a special controller program had been loaded either online or from a Kansas City standard cassette. There was a hard wired writable control store ram diagnostic in this device. This console permitted complete control of a string of 8 disk drive cabinets including the ability of writing arbitary data over your production disk packs. We were very very very very carefull !

Gleaming boards from the BX350 disk pack drive controller.  All that glitters is sometimes actually gold! The view into a full card cage of these was like looking into Aladin's cave.  

This card was one of about a thousand that came from the memory cabinets. The memory chips are intel  4kbits by 1 .  Our Burroughs was maxed out with 8 Megabytes.  The Burroughs had very good ECC and could tolerate a small number faulty chips. We spent a good portion of our time replacing many of these rather forgetfull memories.

Engineering Display cards. These would be clamped onto edge connectors and you slided a lable into the label holder which told you what the lights meant for that particular card. In the B7800 they were used only on the Peripheral Control Cabinets

Core memory plane from a Burroughs DCP, one of about 20 such boards in every DCP


click and zoom in on this image

A PCC cabinet line driver interface card

Console card from the B800  maintenance processor

A view of the Burrough B7800 mainframe and backplane. The backplane was a robot wire wrapped marvel and maintenance nightmare. All done with twisted pair wire, all colored alike. We we had to perform modifications ( hardware patching! none of this download new firmware nonsense) we used yellow wire wrap wire to help remind ourselves where and what we had done. Wire wraps could be up to four levels deep. Dead wires were left in place because it was impossible to disentangle them. Worst faults were intermittant ones whereby insulation would be cut through by adjacent sharp wire wrap pins. The small floating PCBs are clock distribution modules.

The awesome backplane. Source of many intermittant faults and surely the last word in heroic complexity.

Detail from above.  The dust dates from 1989 when this machine was powered down for the last time. This is a clock distribution board, not a delay line as many erroneously thought. Some of those wires are mine.

Some views of the PSU  There were about 40 of these in each mainframe cabinet. They took a raw DC input of 120Volts and a pioneering switchmode power regulator transformed it to 4.8V or -2.0V. The two coils visible here are current shunts for automatic short circuit constant current mode running. You cannot parallel voltage sources, they must have some resistance and  coils of bronze resistance wire provided it.
The primary DC supply was derived from switched SCR rectifiers that were directly connected to large 3 phase step down transformers and these were direct on line.  Everytime the lights blinked we moaned to ourselves about our fates and attended to repair 3 or 4 of these units when the output transistors had been popped by a surge or other transient. We became very good at replacing power transistors in a hurry. A tripped breaker at the front told us the unwelcome news. Often the machine would continue to run with up to six dead PSUs.
burroughs psu images
The many power supplies sat in these racks at the top of the machine, behind the front display panels. The blue sheets are solid aluminum bus bars that carried about 5 kilo-amps of 4.85V and -2.0 V to the card cage. The unit shown here is a negative 2.0 Volt psu that would have joined its forty odd companions in this rack . Each supply could source 135 Amps under normal conditions

A sadly empty card cage

The completely wired wrapped cards, no two were alike. This was not a machine for board jockeys.
Wire wraps were two deep and productions cards were wrapped by robot.
This board and its hundreds of companions was biult from a hybrid mixture of CTuL logic and TTL logic for more rarely used MSI functions. Chip makers were Fairchild and ITT.  CTuL chips were power hungry. They would overheat and destroy themselves in the obsence of flowing air.
The above two images are from one of the hundreds of circuit cards from the Burroughs B7800 CPM and IOM mainframes.  The chip family is the little emulated and not lamented CTuL family that was made on special order from Fairchild and ITT. All the boards were totally wire wrapped with the ICs soldered in place, the chips were never socketed. The wire wrap was two deep per post. The close up shows a clock distribution amplifier, every card had at least one of these. We had to perform chip level repairs on these boards. There were no schematics.  What we had we the so called LOCAL books (Logical and Chip Analysis) which had a typographical tabular convention that specified the logic equations for the board and the complete wire map in tabular form. From test vectors injected by the B800 mainenance processor the machine and the board called be put into a state one clock cycle before the error vector. We traced the faulty logic state back from the front panel, to one or more cards and finally to the chip or wired OR signal. Some horrible wire OR signals had over one hundred terms and could take days  of very pernikity probing and thinking. Intermittant faults were common. When working this way with a suspect board out of the card cages on an extender we would be exposed to the full blast of the underfloor cooling and this was highly unpleaseant, the set point temperature was 12 degrees C.  Cooling failures were potentially diasterous and one of my very first projects was to construct an overtemperature detector and automatic shutdown controller.

CTul  was somewhat like ECL ( see my ECL rant   ) but yet completely unlike it. It was a strange RTL-like logic with open emitter output. It was incredibly power hungry, a chip left powered up without a hurricanes' worth of cold air blasting over it could get too hot to touch. I dont believe that anybody other than Burroughs ever used this logic family. They choose to use this logic family very early and only gave it up when the A10 series mainframe was released make of TTL.  The logic family was quite fast. The B7800 master clock ran at 8Mhz.  I believe that the CTUL logic family was also used by NASA on the Vehicle Flight Controller on the Saturn V rocket.

There were only SSI functions available in the family, but it was sort of TTL compatible and the B7800 used TTL gates for things like bipolar memory. Logic supplies were 4.75 volts and negative 2Volts.  The CTuL gates did not have gain, and that meant that after a logic signal had propagated through 3 levels of logic a buffer or amplifier would be required to regenerate the logic level.  

CTuL first appeared in 1965 and the images below are from an ancient Fairchild manual. It predates TTL by about 10 years. I suspect it was developed to get around Motorala ECL patents.
Historic CTUL data sheet from Fairchild

electrical compliance plate from a type 235 quad density disk pack drive unit. The power consumption was no engineering overestimation it was quite real.  Most of the power consumed was utilized in spinning the 12 fourteen inch diameter platters of the disk pack at mains synchronous speed, the power consumed was mostly the induced drag in air. The air was given a significant centrifugal acceleration and this facilitated both cooling and cleaning the air gap space. The suction induced at the core of the pack would help suck air through the absolute filter that required changing every six months or so.  This particular drive unit had 170 Megabytes of formatted space.

A nd for old time sake,
 here is the CANDE manual, but only on microfiche. No , you cannot read it, my scanner is not that good!
if you zoom in deeply you might just be able to make out the famous railroad diagrams that "explained" the sometimes bizarre syntax.

Burroughs chip house code to commercial equivalents list

These pages are burroughs house part number codes to commercial equivalents. I prepared this list sometime in 1987 to aid myself in sourcing replacement commercial parts. Burroughs then liked to charge $50 for a 7400 quad nand gate because it had their  house code on it...and off course, your mainframe wouldnt work unless it had the genuine burroughs part...wouldnt it now!
I have provided this list as a service to hobbyists and radio hams that may come across discarded computer boards packed with assorted TTL chips that are useless unless you can find the commercial part numbers. The list was constructed largely from schematics, order books and sometimes chip manufactures would print the burroughs house code on top of the chips but also print the commercial type numbers underneath!  This list is not available in machine readable form.
Most of my digital projects documented elsewhere were built with discarded burroughs chips

Memoirs of a mainframe engineer,
my time as a Burroughs B7800 mainframe engineer

front cover, just a teaser
Here is my complete scan of the B6700 systems architecture manual

Here are some images of Burroughs places and people kindly sent to me by Mr M Morreal a former manager at
Burroughs. Here are some of the people behind the Burroughs Large Systems machines
Am image of the Treddyfrin plant
click them to download full res images

Now thats some real history.  Dont forget to have a look at a complete edition of B-Line, and B-Line-2 the corporate journal from the mid sixties

emails welcomed at

my warm thanks to all that have written so far.

mod record: Mon Sep 29 18:52:23 EST 2008 added partial scan of B6700 architecture manual, add link to the B-Line journal    Fri May 29 20:00:35 EST 2009 added image contribution from Mr M Morreal.
Fri Jun  5 19:05:39 EST 2009   added new link to scans of Bline2
Tue May 26 17:23:17 EST 2015  added Cliff Bellamy tribute.