PDP7 Digital Equipment Corporation PDP-7 PDP7
History and Design

Initially conceived as a repackaged PDP-1 with higher module density and faster cycle times, the goals for the PDP-7 range were for a lower cost machine 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 compatibility was added to the design.

A sales target of 120 systems(1) was set for the PDP-7 design, more than all of the other computers DEC had built at that time, however it is not known if this target was ever met. Performance was increased with a cycle time drop from 8uS to 1.5uS (limited by the core memory available at that time). Processor, memory and I/O sections were designed around the new B System "Flip Chip" modules, themselves based on earlier 10MHz system modules. Thus the PDP-7 was born, after the first 50 or so systems 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 8-bit ASCII code rather than 5-bit Baudot. The user panel now 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 the first DEC machine to use automatic wire wrapping for production 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 (serial #1). That process only took 9 months, starting in April 1964 with production units delivered in December 1964, the very first production unit going to Bell Labs (serial #3), being hand built by a DEC field service engineer.

Roughly half way through production the PDP-7, the I/O system was 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 shipped in August 1966.

For a more in depth review of the PDP-7 computer and it's design process, see the 1978 DEC book "Computer Engineering - A DEC view of hardware systems design", by Gordon Bell, Craig Mudge and John McNamara, ISBN 0-932376-00-2.

If you know of any information about any of the PDP-7 systems worldwide, location of existing systems, spare parts, ancillary bits, software, tapes or manuals, then please let us know here. PDP7

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1. Various information on the internet gives sales of the PDP-7 systems as 120 units, however we have not found any information to substantiate this number. 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. To date the only firm evidence for the number of systems produced is the 18-bit Service list, showing 99 systems. Unless further information surfaces in the future, which is probably unlikely, 99 shipped systems it will have to be.
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