Vintage Computing LGP-21

The original LGP-21

Collect ‘em all! Released in 1963, the LGP-21 was the last member of Librascope’s family of serial memory computers: Lower cost, a simplified user interface, and slower than the LGP-30. Designing an FPGA replica is meant to be a “practice run” for me – next, I hope to contribute to the restoration of two real LGP-21s at the technikum29 computer museum!

A low-cost sibling

After the major success of the LGP-30, released in 1956, its big brother RPC-4000 must have been a disappointment to General Precision and its Librascope division. When it was launched in 1960, the bit-serial magnetic drum architecture was already past its prime, and other players took over the midrange computer market with machines based on the brand-new ferrite core memory. But Librascope did not give up on the bit-serial, non-random-access memory design yet.

In March of 1963, the LGP-21 was launched. Compared to the original LGP-30, it used transistor logic instead of tubes, and a lower-cost magnetic disk memory instead of the massive drum. But the CPU logic was very similar to the LGP-30. In fact, programs were binary compatible except for the input/output instructions, which offered more flexibility in the LGP-21 at the price of not being backward compatible.

Librascope LGP-21

The original Librascope LGP-21, with optional paper tape equipment and register display for programmers. Fashionably styled like a sideboard...

Cost reduction was a big focus of the new design, and central to the sales pitch. “The first full-capability computer priced from $16,250” was the advertising headline in the US. (Although the small print explained that you would have to pay another $5,250 for the Flexowriter.) In Germany, the LGP-21 was about half the price of the LGP-30, and was advertised as “the first fully programmable digital computer below 100 000 DM”.

Low cost took priority over performance, and the LGP-21 was actually quite a bit slower than the LGP-30: The bit clock rate was reduced (80 kHz in the US version, tweaked to 100 kHz in Europe, vs. 130 kHz for the LGP-30). And on top of that, the magnetic disk packed two interleaved logical tracks into each physical track – which meant that only 32 instead of 64 read/write heads were needed, but that the average wait time for the next data word or instruction was doubled! So the LGP-21 was slower than its older brother by a factor of 2.5 in Europe, and more than 3 in the US version.

The other big focus was to simplify the user interface. The original US version took this furthest: Only three buttons and a status light were presented to the routine user – enough to power up the computer, re-start the program pre-installed on the magnetic disk, and then make inputs from the Flexowriter terminal. Further controls, needed to load programs, test them in single-step mode, and select program options via “branch control” switches, were discretely hidden behind a lid. The oscilloscope to display register contents, a must when debugging programs, was sold as an optional accessory. At least the programmers will have appreciated its larger display size compared to the LGP-30 and even the expensive RPC-4000!

Librascope LGP-21 control panel

Three buttons and a status light were all the regular user needed to operate the LGP-21. A lid covered additional controls for programmers.

The Eurocomp version

Schoppe & Faeser in Minden/Germany, the licensed manufacturer and distributor of the LGP-30 and RPC-4000, added the LGP-21 to their portfolio as well. By 1964, they marketed all these computers under the “Eurocomp” brand and worked with distribution partners in various European countries.

Schoppe & Faeser took some liberties with their version of the LGP-21 for the European market: Most noticeable, the computer was packaged quite differently, in an enclosure that matched the large RPC-4000. This allowed peripheral devices like fast paper-tape equipment to be sold with both machines. Also, the European user interface did not take the simplification quite as far as in the original LGP-21: While the programmer controls were smaller and clearly separated from the main user buttons, they remained in plain view, and the oscilloscope for register display was always built in, like on the earlier computers.

The European version of the LGP-21, complete with paper tape and magnetic tape peripherals, at the technikum29 computer museum. Photo Heribert Müller,, Creative Commons License CC BY-NC 4.0

The European version of the LGP-21: built under license by Schoppe & Faeser and styled like its big brother, the RPC 4000. Photo courtesy of technikum29 computer museum,, under Creative Commons license CC BY-NC 4.0.

Control panel of Schoppe & Faeser's version of the LGP-21

Schoppe & Faeser had programmers and advanced users in mind: The register scope was always built in, single-step controls are grouped on the side but always accessible, and the five branch control switches are as prominent as the basic start and power buttons.

There were also less visible technical differences: The Schoppe & Faeser LGP-21 operated at a 25% faster clock rate than the US version. And at least one highly complex peripheral unit, a magnetic tape drive, was developed entirely in Germany, in collaboration with tape specialist Assmann in Bad Homburg. One of only a handful of units ever made is on display in the technikum29 computer museum in Kelkheim/Germany.

Various upgrades and options were available for the LGP-21: Interfaces for process control, modifications of the logic equations to enable e.g. fast instruction sequencing and data indexing, and a double-sized memory. For these, it is not clear to me whether they were developed by Librascope or by Schoppe & Faeser. The documentation available today refers exclusively to the Schoppe & Faeser version; see the documentation section for details.