Veteran Microsoft Engineer Crafts Custom PDP-11 RAM on FPGA

Why vintage memory costs remain a barrier

Original memory chips for the PDP-11 were produced in the early 1970s. Over the decades the supply chain has dried up, and the few remaining units command prices comparable to rare collectibles. Collectors and preservationists often face costs that exceed the budget of a hobby project.

Dave Plummer’s journey from Microsoft to retro hardware

Dave Plummer spent more than a decade at Microsoft, where he contributed to the Windows 95 task manager and the Windows Media Player. After leaving the company he turned his attention to classic computer systems, documenting his experiments on a personal blog. His reputation for deep technical insight gave him the credibility to tackle a project that many considered impractical.

Designing a modern replica of PDP-11 RAM

Choosing the FPGA platform

Plummer selected a commercially available FPGA development board because it offers reconfigurable logic that can emulate the behavior of legacy memory cells. The board provides enough input/output pins to match the original PDP-11 memory interface while keeping the design affordable.

Mapping legacy memory cells to modern logic

The original PDP-11 used 16‑bit word cells with parity checking. Plummer wrote hardware description code that reproduces the timing, refresh cycles, and error detection of the vintage chips. By simulating the exact electrical characteristics, the FPGA module behaves indistinguishably from the original hardware when connected to a working PDP‑11 system.

Technical specifications of the custom board

The finished module offers the following features:

Capacity of 64 KB, matching the standard configuration of early PDP‑11 models.
16‑bit data bus with parity generation and checking.
Support for both static and dynamic memory access modes.
Power consumption under 1 W, well below the original specifications.
Programmable timing parameters to accommodate variations in host systems.

All components are sourced from current suppliers, eliminating the need for obsolete parts. The board layout follows the original pinout, allowing a drop‑in replacement for the legacy memory module.

Testing and performance results

Plummer validated the design through a series of functional tests. The process involved connecting the FPGA board to a restored PDP‑11 and running diagnostic software that checks for address line integrity, data retention, and parity error handling.

Boot the PDP‑11 with the FPGA board installed.
Run the built‑in memory test routine from the operating system.
Verify that all memory locations report correct values and no parity errors.
Compare the execution speed with that of a period‑correct memory module.

The results showed identical performance to the original hardware, with the added benefit of configurable timing that can be tuned for newer peripheral devices. The board also survived extended operation without overheating, confirming the reliability of the modern components.

Impact on the retro computing community

By providing an affordable, reliable alternative to scarce vintage memory, Plummer’s project opens new possibilities for enthusiasts who wish to preserve or restore PDP‑11 systems. The open‑source nature of the hardware description files encourages further experimentation and adaptation.

Reduces the financial barrier for museums and hobbyists.
Enables educational institutions to demonstrate historical computer architecture without risking rare components.
Offers a platform for developers to explore low‑level programming on a classic system.
Demonstrates a practical use case for modern FPGA technology in heritage computing.

Plummer documented the entire process on his personal site, providing schematics, source code, and step‑by‑step assembly instructions. The community has already begun to adapt the design for other vintage machines, showing the broader relevance of the approach.

For readers interested in the technical background of the PDP‑11, the Computer History Museum offers detailed archives on the system’s architecture. Official FPGA documentation from Xilinx explains the capabilities of the development boards used in this project. Together these resources give a complete picture of how modern engineering can bridge the gap to historic computing platforms.

Overall, the custom FPGA RAM module stands as a testament to what can be achieved when seasoned engineers apply contemporary tools to solve legacy challenges.