Digital Equipment Corporation PDP-7

History and Design

The PDP-7 was initially conceived as a repackaged PDP-1 with higher module density and faster cycle times, with some of the goals for the new PDP-7 range to be a lower cost machine, but with greater performance than any of the earlier DEC computers.  However when consideration was given to the range of PDP-1 and PDP-4 software, an additional goal of PDP-4 software compatibility was added to the design.

A sales target of 120 systems was set for the PDP-7 design, more than the total of all of the other computers DEC had built at that time, however it is not known if this target was ever met[1].  Performance was increased fivefold with a machine cycle time reduction from 8uS to 1.5uS, that being limited by the core memory available at the time[2].  The processor, memory and I/O sections were designed around the new B series System "FlipChip" modules, themselves based on earlier 10MHz system modules, and so the PDP-7 was born.  After the first 50 systems were produced a design review reworked the I/O systems to use 2MHz R System boards, a design philosophy very much in keeping with the original PDP-1 computer.

Upward program compatibility with the PDP-4 was kept, but using the newly developed 8-bit ASCII code rather than 5-bit Baudot of the PDP-4, the user panel was relocated to the "side" of the cabinet from the "end" configuration of the PDP-4 allowing greater access for servicing.  The PDP-7 computer was an assembly pioneer being the first DEC machine to use automatic wire wrapping for production, ironically using programs developed on a PDP-4.

Design of the PDP-7 cost less than $100,000, excluding modules and staff costs, from initial concept to first prototype (S#1).  That process only took 9 months, starting in April 1964 with production units delivered in December 1964.  The very first production unit (serial #3) went to Bell Labs, it being hand built by a DEC field service engineer.

Roughly half way through production of the PDP-7, the I/O system were reworked to use the lower cost and lower speed R System units, and was re-badged as the PDP-7/A and introduced in 1965.  Thoughts then turned to the next computer in the 18-bit range, to be fully auto wired, better specified, have more memory, be cheaper and cure the "dirty" air cooling problems of the PDP-7.  Initially called the PDP-7/X, it eventually became known as the PDP-9, the fourth in the range of 18-bit computers and was first shipped in August 1966.

For a more in depth review of the PDP-7 computer and its design process, see chapter 6 (page 147-153) of the 1978 DEC book "Computer Engineering - A DEC view of hardware systems design", by Gordon Bell, Craig Mudge and John McNamara, ISBN 09-3237-600-2. Available here, courtesy Microsoft Research.

A DEC publication "Digital at work - snapshots from the first 35 years" is available here[3].

A publication by Bob Supnik entitled "Architectural Evolution in DEC’s 18b Computers" is also available here (©2006 Bob Supnik).

If you know of any information about any of the PDP-7 systems worldwide, options, location of existing systems, spare parts, ancillary bits, software, tapes or manuals, then please contact us.


  1. Various information sources on both the internet and in literature gives the total sales of the PDP-7 systems as 120 units, in our researches however we have not found any definitive information to substantiate this number.  To date the only firm evidence for the number of systems produced is the 1972 18-bit Service list, which shows 99 systems.  Unless further information surfaces in the future, which is now probably unlikely, 99 shipped systems it will have to be. [top]
    1. From the DEC book "Computer Engineering - A DEC view of hardware systems design", it is apparent that the original - sales goal - for the PDP-7 range was 120 units.  Available on-line, courtesy Microsoft Research
    2. The 1978 DEC historical document "Nineteen Fifty-Seven To The Present" copy here, has (on page 6) that "One hundred and twenty PDP-7's were sold altogether", whilst on page 86 in the table of "Installed minicomputer Systems", it lists the PDP-7 as having 115 installed systems at January 1st 1977 with deliveries starting in November 1964, citing the International Data Corporation EDP Industry Report 22nd April 1977.
  2. A find in DECuscope Volume 10 1971, the newsletter of the Digital Equipment Users Society, has unearthed an article by A. R. Atherton of the Cavendish Laboratory, Cambridge, England (PDP-7/A S#126), entitled "MODERNIZING A PDP-7", where a new 16K core store was fitted to their PDP-7/A and the opportunity taken to reduce the cycle time of the computer from 1750 nsec to 875 nsec, doubling its speed.  A supercharged PDP-7 ! [back] [top]

  3. Digital Equipment Corporation were instrumental in some aspects of the then fledgling internet. [back] [top]
    1. 1985 - dec.com was established as a "class B" TCP/IP network.  DIGITAL also created the first corporate Internet mail gateway, giving every email user in DIGITAL full access to the Internet.
    2. 1986 - DIGITAL creates the first Internet firewall and establishes http://gatekeeper.dec.com as a major FTP site on the Internet.  HP Labs explanation of what happened to http://gatekeeper.dec.com
    3. 1995 - AltaVista is the most advanced information search and indexing technology available for the World Wide Web.  The software conducts the most comprehensive search of the entire Web text at speeds up to 100 times faster than spiders used in conventional information search services. [back] [top]

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