Vintage Computing RPC-4000

The LGP-30’s big brother

Imagine working at Royal Precision in 1960. This joint venture of General Precision and Royal McBee is a great place to work: With the LGP-30, they are the market leader in the small computer segment – 450 units sold! Only IBM has a better seller, the IBM 650: A larger system, with close to 2000 units in the market.

The LGP-30 is a magnetic drum computer. It is awkward to program, because the programmer has to work around the constraint of the sequential access to data and instructions on the drum. But it is compact and affordable, using only about 100 tubes. Only 24 of these are needed for the actual CPU logic!

Where to go next? Upmarket, to eat IBM’s lunch! In 1960, Royal Precision proudly announces the RPC-4000: Fully transistorized, which enabled more complex CPU logic; more flexible, scalable peripherals, and – a slow, non-random-access magnetic drum! Which is why so few of us today have heard of Royal Precision…

This page gives some information about the original RPC-4000, and briefly describes my FPGA-based replica – which is still a work in progress at this time.

Advertisement for RPC-4000

The RPC-4000, causing some excitement (at least in this ad from 1960...)

The RPC-4000

The RPC-4000 was structurally quite similar to its predecessor, the LPG-30: While it used transistors instead of vacuum tubes, it was still a bit-serial computer built around a magnetic drum, which served as the main memory. Compared to the LGP-30, the drum capacity was increased, but the speed (bit rate, and hence computer clock speed) was not. Royal Precision implemented several tricks to enable faster program execution.

In contrast to the LGP-30, the RPC-4000 was not a big success in the market. The 1964 BRL market report claims 104 units in operation, and that probably includes customized special-purpose devices. Disappointing compared to the approx. 500 LGP-30 units which had been produced by that time!

The competitors had released new computers with the brand-new ferrite core memory at the same time as the RPC-4000. No longer hampered by the speed of a mechanically moving drum, and with random access to any memory cell at any time, they left the RPC-4000 behind, technologically and commercially. IBM announced the 1401 in late 1959, and according to an earlier BRL report, had installed 2800 units by 1961, at a similar $70,000 price point. DEC’s PDP-1, launched in 1960, only sold 53 units in total, but laid the foundation for many generations of highly successful small computers.

I am aware of only one surviving RPC-4000, and that one is apparently in non-working condition. It is owned by the Computer History Museum in Mountain View, California, but I understand it is in storage, not on display in the collection. The CHM has a brief description here, although with a clearly incorrect date.

An original RPC-4000 installation

The RPC-4000 (right) with a modernized Flexowriter terminal and the standard paper tape punch/reader accessory (center)

Claims to fame

So, why would one be interested in the RPC-4000? A few reasons for me:

Resurrecting the RPC-4000

I found this interesting enough to design a replica based on the same approach as my LittleGP-30: An FPGA to implement the CPU logic and magnetic drum, with a front panel to turn this into a self-contained, handheld gadget. An external computer, connected via USB, replicates the terminal and paper tape units.

Handheld FPGA-based replica of the RPC-4000

The replica is based on a Spartan-6 FPGA board, mounted behind a small, handheld front panel.

Closeup of replica display

A color LCD replaces the oscilloscope display for the CPU registers. The rotary selector to the right of the display is actually present in the real RPC-4000, to select one out of eight accumulators for display. As are the two switches below the front panel, which can write-protect critical drum tracks.

This project is still work in progress, and hence I do not have building instructions or project files online yet. The current status, as of November 2018:

The quest for software

A decent selection of scanned manuals for the RPC-4000 is available e.g. in the Bitsavers archive. The archive also comprises a set of “program writeups” which document a few library routines and application programs – including Mel Kaye’s Blackjack program. But none of the actual software is available online.

So I embarked on this project with two leads to RPC-4000 software, but knowing full well that I might end up with a computer replica without any software to run on. And indeed that risk has caught up with me…

So, here I am – plenty of manuals, a largely working replica, but no software yet. If you have any leads to paper tapes or other forms of software for the RPC-4000, please drop me a line!


I don’t have building instructions for the RPC-4000 yet, and the FPGA code will undergo some further tweaking. In the meantime, here are some very preliminary documentation and the current FPGA binary: