![[11/20 front panel]](11_panel_2.jpg)
Created Oct 7 2007. Last edit Dec 5 2008. Herb Johnson
Introduction:
Introduction to the Greenough 11/20 collection
MARCH/Infoage PDP-11/20 exhibit
MARCH, VintageTech and InfoAge
Questions about the system and collection:
What does MARCH have?
How was it used and programmed?
How was the 11/20 operated?
Can the 11/20 be restored and run today?
References:
Web links for more information
Contributors to the exhibit
Exhibit history
Other DEC systems at MARCH
In August 2007, the MARCH vintage computer club acquired a PDP-11 system of some sort. When I came by to help Evan set up rooms for new exhibits and storage, I saw the system and saw that it was an 11/20, with a largely COMPLETE collection of manuals, schematics, and even the paper-tape operating system. I saw right away that this could be the basis for an exhibit of 1970's "state of the art" for DEC minicomputers, even without running the system. I also saw that I could create this exhibit for MARCH, as an example of a historic and documented collection as well as an example exhibit. After some consideration and encouragements from MARCH members, Evan agreed and I began work.
I soon found that this system was part of document scanning systems developed by the US Government's National Bureau of Standards (NBS, now NIST) from the 1950's through the 1990's. This particular system was donated by M. Leighton Greenough, who developed systems used those systems at NBS and later as a consultant. He eventually purchased it and continued to do similar scanning work for the US government. Mr. Greenough was kind enough to write a description of his history with these system and provide photos, which are now available on this Web page. This particular PDP-11/20 was used for evaluation of experimental scanning techniques and not for production scanning.
This 11/20 donation includes the original 1969 distribution of DEC's paper tape operating and development system, including paper tapes and manuals. A description of the DEC paper tape software system and its console of switches and lights, is on the Web pages linked in this section. Documents, boards, paper tapes, the contents of the 11/20 itself, were listed and photographed. The history of this and related systems at the NBS, with photos, was provided by Mr. Greenough himself. Others provided related artifacts, and videos to display operation of a papertape programming environment like the 11/20's. I produced a set of Web pages (this is a subset) and the exhibit posters and the exhibit displays.
- Herb Johnson
On September 13 and 14 2008, the MARCH vintage computer club had a computer show "VCF-East", and the public opening of their new Museum facilities at InfoAge. The PDP-11/20 described here was one of the Museum exhibits. My thanks to several people for their work and contributions. The posters in the exhibit are based on the contents of these Web pages. The 11/20, some artifacts, and videos of computer operations were also on display.
Here is a representation of the MARCH PDP-11/20 exhibit, as a set of HTML and image documents. That Web page shows images of the artifacts and posters for the exhibit. The images have links to larger images, documents, etc. of the actual exhibit. Videos included with the exhibit in September may not be available.
In October 2008, MARCH was given copyright and a copy of all work done on the 11/20 exhibit. (I have a non-exclusive license to use the work I did for MARCH on this exhibit.) MARCH will be responsible for any further Web pages for that artifact, the exhibit and the archives and artifacts. We will provide links at this site (at retrotechnology.com) when MARCH provides Web links.
MARCH is the Mid-Atlantic Retro Computing Hobbyists, with facilities at the InfoAge Science Center in Wall, NJ. The MARCH Web site is at this link. To contact Herb Johnson, check my home page at retrotechnology.com.
MARCH/InfoAge and the PDP 11/20 Exhibit
MARCH, VintageTech and InfoAge
The InfoAge Web site is at this Web link.
This is the Web page for the "Vintage Computer Fest - East 5.0"s how as sponsored by VintageTech
The MARCH Yahoo! maillist is at this link.
Questions about the system and collection
![[11/20 system and docs]](dec_n_docs.jpg)
Here's how Evan Koblentz, MARCH Board Director, described the acquisition, including details from the original owner:
"We're very lucky: the guy who donated it sent me an interesting blurb about the machine's history. He emailed me on June 27, saying that he learned about MARCH at the recent Trenton show." He dropped off the system early in August. Here's his email: - Evan Koblentz.
"This machine was bought by the National Bureau of Standards in the early 1970s for a project I was working on. I modified it a bit to create an interface to a flying spot scanner. After some use it was declared surplus upon my retirement in 1976. A year later the U.S. Postal Service Research Laboratory acquired it and commissioned me to modify the scanner for their work. A memory expansion unit was there added. In 1986 the USPS no longer needed the equipment and accepted my unsolicited bid for purchase of the entire system, including the computer, scanner and other associated equipment. The computer has been on a shelf in my house ever since. Now I would like to find a good home for it." - Leighton Greenough
In a later communication from Greenough, he affirms that "although incorporated in a system compatible with [Census] specialized documents, [this computer] had no connection with [the Census Bureau] beyond evaluation of experimental scanning techniques."
Components available and condition (from Greenough)
Main chassis without power supply , in rack mounting
Power supply for above, H720e
Expansion memory: Plessey 1116 B, Core (16k, 16-bit words)
Bus cables: 6, 9 and 15 feet
Boxes, approximately 15x22x4 (2) of manuals
Boxes of DEC (in trays) and custom programs
Miscellaneous DEC circuit boards
There is no Teletype console
Main computer chassis is in apparently excellent condition. The power supply (included) has been separated from the main chassis for convenience in handling.
Three boxes of paper tapes (two sets of DEC system stuff, some DECUS stuff);
two boxes of manuals (about two sets)
box with core memory module, Plessey Microsystems PM-1116-B ? with manuals
box with Unibus cables
white binder of manuals
two external power supplies (one in box)
The boxes have old lables such as "manuals from NEWMAN, 1977"; "paper tape programs from USPS Mach. FOSDIC VI". As of Aug 3 2008, the boxes have yellow Post-it notes saying "11/20 Greenough".
A detailed but partial inventory of the hardware, software, manuals and paper tapes was done and made available on the Web in 2007-08. That information is now up to MARCH to display and use.See MARCH's Web site for more.
![[inside of 11/20 bus]](11_cardcage2.jpg)
<-----------(bus cabling, M920 jumper blocks, resistive terminators) ---------->
G225 M1091 G225 M1091 M825 M823
H207 G226 G226 M7290 H207 G226 G226 M7290 M792 M780 M824 M822 M224 M820 M725 M724 M726 M728 M727
YA
G102 G102 M7290 G102 G102 M7290 M792 M780 M824 M822 M224 M820 M725 M724 M726 M728 M727
YA
G102 G102 G102 G102 M792 M105 M225 M821 M224 M820 M725 M724 M726 M728 M727
YA
H207 G226 G226 G103 H207 G226 G226 G103 M792 M7820 M225 M821 M224 M820 M725 M724 M726 M728 M727
YA
PDP-11/20's like this system, with optical and other equipment, was developed and used by the NBS in the 1970's, to scan documents for the National Census. In the 1980's they did similwar work for the US Postal Service. This particular computer was used for development and experimentation. A description of the history of the NBS FOSDIC system line, with photos of use, was provided by the original developer. That history was put on one of the developed Web pages of the exhibit for MARCH. A subset of that Web page is at this link.> See MARCH's Web site for more.
The NBS or National Bureau of Standards, is now the National Institute of Standards and Technology (NIST) at nist.gov. NBS and NIST research provided technological leadership in many areas, and also established and maintains standard measures. A general description of how they developed massive optical scanning technology such as FOSDIC for the Census Bureau is at this link.
Programmers studied DEC's "PDP-11 Handbook" to program the 11/20, to operate its elaborate console or front panel, and to use the programming tools sold with it. Exerpts from that handbook were typed and provided as Web pages for the exhibit. See MARCH's Web site for more.
This chapter is the Introduction
The components of the PDP-11 were in this chapter
Use of the front panel or console was described in this chapter
This chapter described the Paper Tape Operating System, the manuals and paper tapes which are in our collection. See the "inventory" for a list of them.
To program the 11/20, the minicomputer had to be powered up, and programs loaded in. Initial loading, as well as diagnostics and tests, were done through the front panel. Then programs were loaded from paper tape or hard disk. The computer was often operated from a Teletype, a printing terminal. The section on "operation" describes these methods and devices.
The 11/20 computer itself has no programs in it, not even programs to read programs from devices like the Teletype or disk drives. To load the first programs when the computer is started up, the "console" front panel was often used; the console is also a diagnostic tool for repair and maintenance. A small program called a "boot loader" could then load other programs from those devices. Programs had to be loaded in, one at a time; the program was run, maybe producing a result which was output. Then another program was run, and another, and so on. For applications like the FOSDEC, where the same program was always used, the "loader" programs would be stored in ROM (fixed memory), and they'd load the appropriate programs from disk.
![[paper tapes]](exhib_tapes.jpg)
Otherwise, for program development, the PDP-11/20 was often sold with a Teletype, a printing terminal with its paper-tape reader and punch. Optionally the 11/20 could have a Remex brand high-speed paper tape reader and punch. "Paper tape" contains data and programs; it has sets of 8 holes in rows, and are read or punched by rows to represent "bytes" of data or a single character of text in "ASCII" code. In the era of the 1960's, before hard drives were common and before floppy drives were available, paper tape was sufficient to hold modest amounts of data and programs.
DEC provided a "Paper Tape Operating System" with this PDP-11/20 from 1969-70, as discussed above. Later, DEC developed other operating systems and programming languages for the PDP-11 series.
The PDP 11/20, like many minicomputers of the 60's and 70's, had a "front panel" called a console which was used to access the computer and program it at the binary (bit) level, one memory location and one instruction at a time. Our 11/20 console is described in detail in the "console" manual exerpt described above.
David Gesswein produced a video of starting up a similar computer, the PDP-8. Here's a Web link to my description of that video, which has a link to his video on his WEb site.
![[tty session]](video_ttysession.jpg)
It was common to operate the early PDP 11's with a Model ASR-33 Teletype. This was a person-operated printing terminal with keyboard, which could also read and produce paper tapes. As described above, these tapes provided data and program storage. The Teletype printer is controlled by either the paper tape reader, or data input on the Teletype serial line. The Teletype keyboard, or the Teletype paper tape reader, sends data output on the Teletype serial line. "Serial" means the binary data is sent on a single wire, bit by bit and byte by byte, in a sequential or "serial" fashion.
For the exhibit at MARCH, David Gesswein of pdp8online.com produced videos of a Teletype operating as described above, but with a DEC PDP-8 system. Those videos and descriptions are on his PDP-8 Web site on this Web page. Thank you, David, for your work.
On his Web site, check the video "PDP-8/I Disk Monitor System Session. This linked document is a description of what David is doing in that video, step by step, to run a program using paper tape and the Teletype, on his PDP-8. This is similar to what would be down with the PDP-11.
Other videos on that Web page show paper tape handling - reading and writing - on the Model 33. The model 33 can "read" the paper tapes it punches, or other tapes. The tape is fed over the reader, which has a spiked wheel to pull the tape over the reader. The small line of holes in the center of the tape are the "feed holes". When the tape contains text, Teletype printer can print that text. The model 33 Teletype can also write or "punch" a paper tape with an ASCII or binary pattern of eight bits at a time.
The operator uses the Teletype's keyboard, printer, tape punch and tape reader to interact with the PDP-11/20. The Teletype prints text, either as received from the serial line, or "echoed" from the keyboard, or from ASCII text read from the paper tape reader.
It takes considerable skills, electronic tools, and experience to restore a forty-year-old computer to full operation. That includes knowledge about technolgies no longer in use, and skills to repair digital and analog electronics down to resistors, capacitors, transistors and integrated circuits. One example of such work today is David Gesswein's PDP-8 collection of running computers. Here is a link to an example of one of his repairs..
DEC was purchased by Compaq Computers. Compaq was purchased by HP. In 2008, all I could find on HP's corporate site, was references to Compaq. Some of their pages reference DEC systems of the 1990's and later, which are or were supported "recently".
There are many Web sites devoted to DEC's minicomputers of the 60's and 70's and beyond including the PDP-11 line of minicomputers. They offer far more support and description than these Web pages. A Web search will find them.
Thanks to David Gesswein of pdp8online.com for permission to use these videos, which he produced. He created new videos and text descriptions in 2008, to support this exhibit and Web page. The PDP 8/I and ASR-33 Teletype are owned, maintained and described by him. These and other videos are on his Web site.
There is a Web archive site which links to other archive sites which have manuals on-line. The MANX Web site has its own database, to provide links to some of the on-line manuals for the PDP-11/20.
Some Web sites with information about DEC boards such as those in the PDP 11/20 are:
a field-guide list of DEC modules
by function is at this site
and the module list is also at this site
Schematics per board are at the pdp12.org Web site.
Thanks to Leighton Greenough for his generous donation of the 11/20 and materials, for his subsequent descriptions and photos of FOSDIC history, and his suggestions about the contents of these Web pages. He corresponded with me in 2008 and provided the bulk of information about FOSDIC and the immediate history of the PDP-11/20. Thanks for his contributions, encouragments and good will.
Thanks to David Gesswein for developing new videos of his PDP-8 and Teletype model 33 in operation. Those videos are used here and in the exhibit with his permission. He also loaned a PDP-11 card and paper tapes for the exhibit. David made considerable efforts to produce these videos and to discuss them with me.
Thanks to MARCH member Bill Degnan, for providing links to his 11/40 videos. He provided useful review of this work. Jeff Jonas provided review and discussion of the "minicomputer" poster, and contributed to its content.
Other persons who provided comments and suggestions include Jack Rubin, Evan Koblenz, John Allain, Jim Scheef. Thanks for their interest.
These Web pages and poster contents were otherwise developed by Herb Johnson of retrotechnology.com, from Aug 2007 through September 2008. Much of this material will be supported and hosted by MARCH after that period, including a change of Web site.
It may be informative to see how an exhibit is put together. Here's my notes about this exhibit. - Herb Johnson
Herb Johnson late Aug 2007: Here's a straightforward proposal for a "static" exhibit. First off, the computer would NOT be powered up or running - that may come in time, may not. It will be a "static display" of the system and its documents, as a "here's computing of the 1970's" exhibit. It shows what could be done without hard drives or even floppy drives! And, how minicomputers were used in the period.
Most of such an exhibit would be the items themselves; the rest would be photos and interpretive text to describe use and so forth. Some wall space for poster boards with text and photos, and space for the computer and samples of its documents, paper tapes, and so on. THAT's the basic idea, nothing elaborate, and nothing needs to run. Maybe, with interest, we can get some guys to get a '11 to light up, run some code.
I said "I will take some time to develop this idea, write some text, cut up some photos, lay it out. That can be done in a few days over a few weeks, I will do it on a (non public) Web page on my site. MARCH can look it over and we can talk about it and show work as it occurs." This Web page is a result.
The initial tasks are to inventory what we have in more detail, with more photos. And, to gather more info about how it was used by the Post Office and NBS. That dovetails very well with "computing of the 70's", as I had hoped. Lucky us!
Early Oct 2007: A preliminary list of items is complete, but needs to be verified. A photo survey of the cards and inventory has been done and is on-line. The card list should be identified by function. We also need to know how to connect the power supply, and an apparent serial connector needs to be identified.
Feb 2008: Mr. Greenough mailed to Herb Johnson a CD of images, some nice glossy photos, and a four-page text and cover letter. The text describes his experience with and history of FOSDIC development. That material was scanned and lightly edited. MARCH is discussing how this 11/20, its documents and tapes, the materials from Mr. Greenough, and information on programming, can all be put into an exhibit for the MARCH September 2008 show.
May 2008:Added notes and exerpts from the DEC PDP-11 Handbook. InfoAge now has an HP DesignJet 500 printer which can make professional looking color posters. The PDP-11 cards can't be displayed inside the cabinet, the top of the cabinet only shows a backplane - a photo of the cards will be put on top, instead. [Note: this never occurred.] Mr Greenough provided more information about the FOSDEC scanner optics, which was added to this Web site; and related notes.
June 2008: videos from David Gesswein, made for this exhibit, added to this site.
July 2008: Prototypes of the MARCH Museum exhibit posters for the 11/20 were completed and discussed. MARCH plans to have a computer show and the public opening of their new Museum facilities in mid-September.
Aug 2008:A set of HTML documents were completed which show the proposed exhibit, posters (also as Web documents), and artifacts. The boxes of the Greenough collection were tagged with yellow Post-it notes. Copies of the inventory were placed in some boxes. Better photos of front panel and cards put into Web page and in poster document. Paper tapes and DEC DR11-C provided on loan for exhibit by David Gesswein. Word files given to MARCH to print as exhibit posters.
late Aug 2008:Work on the exhibit and these Web pages completed. Final review and fixes.
mid-Sept 2008: Exhibit posters printed, displayed with PDP-11/20. Paper tapes and displayed DEC card put in glass-covered boxes to display. Linux-based PC set up to show videos of Teletype and PDP-8 to show "operation" like the PDP-11's. Exhibit opened September 15-16 2008 as part of Vintage Computer Festival- East.
A list was compiled of other DEC systems held by MARCH as of mid-2008. That included a few PDP 11/05's, a PDP-8, A 11/23 graphics system, a VAX 750, and an 11/40. Details and photos were given to MARCH in October 2008; contact them for more information.
To email MARCH or InfoAge, see the MARCH Web site as described at the top of this Web page. To contact Herb Johnson, check my home page at retrotechnology.com.
Copyright this document: © 2008 Herb Johnson, from materials now copyrighted © MARCH 2008.
