Questions about the system and collection

How did MARCH acquire the PDP-11/20?

[11/20 system and docs]Here's how Evan Koblentz, MARCH President, 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.

What do we have?

PDP-11/20 system with a 1/20/72 list of boards on the back as follows:

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 is on this linked Web page. It also has a description of 2008 correspondence with Leighton Greenough about the exhibit and Web site. He contributed text and photos which were included on these Web pages and on the exhibit posters. Thank you.

Card positions in 11/20: front panel to far right

[inside of 11/20 bus]

		<-----------(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 is now available on this Web page. Further Web pages references for FOSDIC are on that Web page.

The NBS or National Bureau of Standards, is now the National Institute of Standards and Technology (NIST). 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?

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. Here's some exerpts from that handbook:
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. Photos and discussion are in this chapter exerpt from the DEC 11/20 User's manual.

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.

Absent an operating 11/20 system, we provide a 3MB .AVI video of the operation of a DEC PDP-8 console. The PDP-8 is a smaller "cousin" of the PDP-11, similar in age and function but which runs different software. The video shows the operation of that console, in a manner similar to the 11/20's console, to start up the computer.

A description of the front-panel operations in the above video is in this Web page.

Operation of the 11/20 after startup: the Teletype

It was common to operate 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. "Paper tapes" were described briefly as part of the operation of, and software for, the PDP-11/20. 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

Complete operation of a Teletype with a minicomputer is shown in this 3-minute, 21Mb AVI video. Additional videos as below, will show details of the Teletype's functions. The operator uses the Teletype's keyboard, printer, tape punch and tape reader to interact with a PDP-8, an older DEC computer than the PDP-11/20. The operations shown are similar to those with the 11/20 with the same Teletype. A detailed description of the user's operations in this video is in the linked text file.

Operational features of the Teletype: print text, read and write punched tape

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. A 3MB .AVI video of the printer is below; the text is a listing of a FORTRAN program.

teletype_print.avi(video AVI file).

The model 33 Teletype can also punch a paper tape with an ACSCI or binary pattern of eight bits at a time. A 2Mb .AVI video of the Teletype punching a tape is below. In the audio, you can hear both the punch and the printer in operation. Note the paper tape has rows of eight holes plus a small hole called the "feed hole", which is used to align or feed the tape.

teletype_punch.avi (video AVI file).

The model 33 can "read" the paper tapes it punches, as shown in the 1MB .AVI video below. 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".

teletype_reader.avi (video AVI file)

These videos were produced and provided by David Gesswein of pdp8online.com and are used on his PDP-8 Web site. Thank you.

Can the 11/20 be restored and run today?

It takes considerable skills 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..

MARCH member Bill Degnan acquired a PDP-11 in 2008, and is learning how to work on it. Check his blog Web page on vintagecomputer.net for details about Bill Degnan's PDP-11/40,including UTube videos.