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Mar 31, 2014

Digital's DECSYSTEM-20 - Part 1 - External Parts

As a younger engineer I used to use some Digital's multiuser timesharing computers for program development but never actually studied them very deeply at that time. So I might research one such a computer and share my findings with you.

digital logo

Since this text is mostly a study of different documents of that time (and later) it is mostly borrowed from various sources (see REFERENCES at the end).

If you are more interested in the internals of the DECSYSTEM-20 (PDP-10 with a KL-10 processor) then jump to the pert 2. of this article since this 1. part is mostly about peripheral parts of the system.


The DECSYSTEM-20 was primarily designed and used as a small mainframe for timesharing. That is, multiple users would concurrently log on to individual user accounts and share use of the main processor to compile and run applications. Separate disk allocations were maintained for all users by the operating system, and various levels of protection could be maintained by for System, Owner, Group, and World users. A model 2060, for example, could typically host up to 40 to 60 simultaneous users before exhibiting noticeably reduced response time.

A DECSYSTEM-20 computer, behind the doors on the left floppy disk drives and right the console

The DECSYSTEM-20 was a 36-bit Digital Equipment Corporation PDP-10 mainframe computer running the TOPS-20 operating system (Products introduced in 1977).

Early KL10 (DECSYSTEM-10) with tall cabinets and the front end  processor cabinet (closest) on "wrong" side. In later models that was moved far to the left so that UNIBUS cables would be shorter

PDP-10 computers running the TOPS-10 operating system were labeled DECsystem-10 as a way of differentiating them from the PDP-11.

Singer is more known about its sewing machines today

Later on, those systems running TOPS-20 (on the KL10 PDP-10 processors) were labeled DECSYSTEM-20 (the block capitals being the result of a lawsuit brought against DEC by Singer, which once made a computer called "system-10").

A DECSYSTEM-20 mainframe computer (PDP-10 KL10 running TOPS-20)

The only significant difference the user could see between a DECsystem-10 and a DECSYSTEM-20 was the operating system and the color of the paint. Most (but not all) machines sold to run TOPS-10 were painted "Blasi Blue", whereas most TOPS-20 machines were painted "Terracotta" (often mistakenly called "Chinese Red" or orange; the actual name of the color on the paint cans was Terracotta).

A false color DECSYSTEM-20 repainted afterwards to resemble newer VAX computers

Digital made 36-bit systems from about 1964 (starting with PDP-6) until 1983 when it was noticed that PDP-10 was competing with the 32-bit VAX computer from the same company. The Jupiter project was cancelled 1983.


Additional to the actual computer cabinets the system consisted of several Input / Output devices, mostly slow terminals, readers, punchers and printers; or high speed disk drives and magnetic tape units.

Typical room of dump terminals that were used for text work since graphics and multimedia needed too much computer resources for that time

Maybe some remote terminals were connected via audio modems. Remote stations were connected using separated UNIBUS and an additional front end processor (PDP-11/40).

DECSYSTEM-20 was built into three cabinets numbered 1 to 3 from left to right. The first one housed the floppy discs and the console with the front end processor PDP-11/40 below it in a drawer. The second cabinet had all the KL10 CPU's I/O logic in it and the third cabinet had the actual KL10 processor with (or without) its memory.

See the following block diagram of a typical system. The blue boxes in the diagram are the three computer cabinets seen in the above picture.

DECSYSTEM-20 block diagram

Faster devices, like disk devices and magnetic tape units, were connected using the MASSBUS. The system always had at least two processors, the main processor KL10 and the front end mini computer PDP-11/40. There could be more front end processors for different purposes.

PDP-11 mini computer (basically the other half of the mainframe DECSYSTEM-20)

If you remove all parts in the block diagram to the left of the dotted blue line (cabinets 2 and 3), the remaining system is the mini computer system PDP-11/40 with its own disk drives and everything. The front end UNIBUS was also connected to the disk drive and remote station so it was really a stand alone capable computer of its own.

DECSYSTEM-10 1080 / 1090 with its peripheral devices

The KL10 processor was started with the assist of the PDP-11/40 front end computer. The PDP-11 was booted from a dual-ported RP06 disk drive (or alternatively from an 8" floppy disk drive or DECtape), and then commands could be given to the PDP-11 to start the main processor, which was typically booted from the same RP06 disk drive as the PDP-11. The PDP-11 would perform watchdog functions once the main processor was running or blink the panel lights of the console.

Video of PDP-11/40 panel lights (same as the KL10 panel but different color). The KL10 panel was connected only to the front end PDP-11 processor directly but could serve the KL10 also

The later KS system used a similar boot procedure. There an Intel 8080 micro processor loaded the microcode from an RM03, RM80, or RP06 disk or magnetic tape and then started the main processor. The 8080 switched modes after the operating system booted and controlled the console and remote diagnostic serial ports.


The most first computers (1940's and 50's) used Teletype terminals or switch panels to communicate. A teleprinter (teletypewriter, Teletype or TTY) is an electromechanical typewriter that can be used to send and receive typed messages from point to point and point to multipoint over various types of communications channels

Video about DEC tape dublication

Teletypes existed already before World War I in USA and have basically nothing to do with computers.

DEC paper-tape format

Teleprinters were later adapted to provide a user interface to early mainframe computers and minicomputers, sending typed data to the computer and printing the response. Some models could also be used to create punched tape for data storage (either from typed input or from data received from a remote source) and to read back such tape for local printing or transmission.

Teletype terminals used also paper tapes

The more 1970's style LA36 DecWriter terminal was a step up from a teletype.

LA36 paper terminal in front of early model KL10

Paper terminals were used in applications where one needed some kind of a document of what had been done with the computer. Typically an operator of the system used such a device. There you could then for example see when the system had been booted up (which was those days rather often). Since the processors did not have any protected modes or memory areas any wrongly acting program could mess up the main memory and crash the OS (operating system). That was a stop and crash for all the users and the operator had to boot it up again. The modern day luxury (protected OS) like Windows which can handle hostile programs was something futuristic those days. Actually the protection mechanisms are in the processor and used by the OS.

LA36 DecWriter terminal

The following video shows LA36 paper terminal in action. Since they were rather noisy and slow they were soon replaced by CRT video terminals.

Video about DEC LA36 DecWriter II

The DEC's VT100 model terminal shown in the next picture was already a rather advanced CRT terminal. It had its own internal microprocessor and that could do some basic block graphics also but it was mainly used for text work, editing programs with text editors, commanding the CPU to compile, link, and run them. Some applications used those for data entry and visualizing results.

Digital's advanced VT100 CRT terminal that had an Intel 8080 micro processor in it

The following video shows that it was all B/W and simple block graphics and ASCII characters. Even international character sets were more or less unknown at that time.

Video about VT100. Since VT100 was very advanced it could show some block graphics but it was mainly used for text processing and commanding the computer.

Punched cards were still used during late 1970's but mostly replaced by CRT terminals and large magnetic disk drives. You did not need to store your data in paper cards or tapes since magnetic disc drives or tapes could store them.

A stand alone card punch station

A typical punched card could store about 80 characters of data: numbers or text or similar.

Video about stand alone card punch

Most often one card represented on line in a text file and so the ordering of the cards was critical. Once you had your card deck ready you could feed it into the computer using a card reader.

Digital's CR10-F card reader was originally manufactured by Documation (model M200) and used by many computer manufacturers, for example Wang.

Video about DEC's CR10-F card reader

Late 1970's paper tapes were no more used in large systems with magnetic storage but in smaller systems it was still useful, for example CNC machine control programs were stored in paper tapes.


Faster devices, like disk devices and magnetic tape units, were connected using the MASSBUS. That allowed a fast speed peripheral device to be connected up to 60 ft away from the I/O cabinet.

RP06 disk drive was originally manufactured by Memorex (model 677)

Memorex 677 disc drives without RP06 side boxes where the Digital MASSBUS specific electronics was situated

A RP06 disk drive which was typically used in a mid 1970's DECSYSTEM-20 KL10 could store up to almost 200 Mb of data. The disc drive was manufacture by Memorex (model 677) with a Digital specific MASSBUS electronics interface card in it. The Memorex 677-II most likely was an IBM 3330-11 clone. They both used 3336-11 compatible 200 Mb disc packs.

RP06 disks behind TD-1 in front of a DECSYSTEM-20 (TD-1 was a high performance 32-bit computer running TOPS-20 from XKL Systems Corporation, California)

The 3336-11 disc pack had 12 discs (of which 10 was used) and 19 surfaces. It was removable since none removable "Winchester" hard discs came later.

3336-11 disc pack 200 Mb of storage capacity

The RP06 disc unit weighted 550 lbs (250 kg) and the disc pack about 20 lbs (9 kg).

Memorex Mark XI Disc Pack (IBM3336-11)

The following video shows how IBM discovered the magnetic disc drive during 1950's.

Video about IBM discovering the random access magnetic disk drive

The TU45 magnetic tape unit is shown in the following picture and video.

TU45 tape units next to DECSYSTEM-2020

Video about TU45 Tape Drive

Tape was mostly used to store backups and its importance in the operation of the computer is and was smaller. Even today most operating systems live around a disc an so they are called DOS (Disc Operating Systems). The reason is of course the direct access to any data in the store which is only possible with disc drives.

[Next part of this article (Part 2) will describe DECSYSTEM-20 internal parts in more detail]






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