<-----------(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

How was it used? - the NBS, the Census, and the Post Office

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.

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.

How was the 11/20 programmed?

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.

How was the 11/20 operated?

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.

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.

Operation of the 11/20 at start-up: the front panel

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.

Operation of the 11/20 after startup: the Teletype

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.

Interactive operation of a DEC minicomputer with Teletype

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.

Can the 11/20 be restored and run today?

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..

That said, these 1970's computers were designed for a long life of use and for repair to the component level. In the 21st century, many DEC computers of this vintage are restored to operation with surprisingly little component work needed. Boards become available through online sales and offered in DEC-related discussiong groups on the Web. It's still a challenging task to restore and operate one of these computers.

Sometime in 2012-13, members of MARCH cleaned up the 11/20 and made it operate as shown here, no peripherals added. No serious damage to components had occured. HOwever some faults occurred and the 11/20 was not fully functional. It's likely more attention to it, could restore it to operation, and peripherals added for use.

Web links for more information on PDP-11's and DEC minicomputers

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 a mirror of the former pdp12.org Web site.

Contact info

To contact Herb Johnson, check my home page at retrotechnology.com.

Copyright this document: © 2023 Herb Johnson. Other 11/20 exhibit materials were copyrighted © to the former MARCH organization in 2008, as produced by Herb Johnson at that date; with rights of use by Herb Johnson. The MARCH organization ended and assets donated to the Vintage Computing Federation Inc.