I often say that we live in a golden age of vintage or retro computing. Countless communities have sprung up to support seemingly all possible old hardware platforms, making DIY replacement parts, and even developing new software for hardware decades old. And virtualization has advanced to the point that so many classic platforms can be run as virtual machines on modern hardware, simulating the usage of one computer on another.
These retro communities are almost always directly proportional in size to the former popularity of the system in question. You have no challenge finding a Commodore 64 users’ group today. And you will have no shortage of modern replacement parts for your Apple IIe. You likewise have a surprising amount of activity today in communities for PDP computers of the 60s and 70s.
But dial back to the 1950s and computing was a significant budget line item for any business. Home computing was decades away and even at the commercial level, only the largest organizations could afford computers. This is one reason why you don’t find many UNIVAC user groups today. Not only is the hardware far too large and power-hungry by today’s standards to be able to practically operate for mere enthusiasts, but there were relatively few of these systems. Most were lost to the inevitable cycle of obsolescence and upgrade cycles.
So I had been happy with my replica vintage 60s and 70s PDPs blinking peacefully at me from my bookshelf, knowing that I most likely wouldn’t be able to capture an even earlier slice of computing history in a similar way. Then I found the LittleGP-30 project.
Jürgen Müller is a man of eccentric, but satisfyingly geeky, tastes. Among his projects on his aptly title e-Basteln (e-handicraft) website are an Arduino-based optical paper tape reader, a recreation of the first commercial mouse by Telefunken, and a DIY scanning-tunneling electron microscope. NBD.
Jürgen also has healthy (?) interest in 1950s computing including the LGP series of computers. LGP (Librascope General Precision) was a sort of early tech conglomerate that evolved into making commercial computing equipment. The LGP-30 was the company’s first “desk computer” in the sense that it was roughly the size of an average office desk, which was a big selling point and kind of wonder when most computers still occupied entire rooms. But it was also designed to be used by a single person unlike timesharing and batch mainframes of the day. In some ways, it can be thought of as a very distance ancestor of the business personal computer.
Slow and limited when compared to the UNIVAC systems of the day, it was affordable for medium-sized businesses both in terms of cost, but also operational expense and sheer size. And its design was crazy-optimized using only 113 vacuum tubes in total with magnetic drum serving not only as working memory, but also as internal register storage and timing signal control. And unlike the PDPs and other computers of the next decade, there was no bank of lights showing the internal state of the computer. Instead, a tiny oscilloscope was mounted into the frame of the LGP, showing the state of its three internal registers through a mask.
Jürgen’s replica is a sight to behold, recreating the operator’s console of the computer with all the colored, backlit buttons, with a small LCD replacing the oscilloscope. A Xilinx-based FPGA runs a simulation of the LGP, while a terminal connection to a PC serves as a stand-in for the teletype that would have been used for program input and output. And as a bonus, he includes a HDMI connection that outputs a visual representation of the memory drum so you can see not just the internal registers on the oscilloscope/LCD, but you can see the actual memory contents of the drum update in real time as you load and run programs.
Building the kit, I didn’t just learn about the hardware of the era. A shocking amount of history and software has also been preserved. There are original manuals, photos, scans of diagrams and schematics, and even company newsletters from the manufacturer preserved and available to read today. The software is comparatively primitive, but there was a lot of it with examples of making the most of the limited hardware and performance.
And the LGP-30 figures large in the Story of Mel. If you have not read it, go and look up this piece of hacker lore. As it turns out the eponymous Mel was an engineer at General Precision. The well-known story is of his hacking prowess and elegant solutions when attempting to port software from the LGP-30 to its successor the RPC-4000.
I highly recommend this simple build to anyone who has an interest in the very early days of commercial computing. I never thought I would be able to use and touch much less build my own piece of this early history, replicated or not. Like I said, it is truly a golden age.
Building
Building the LittleGP-30 was a new challenge for me. While it is not complicated, Jürgen doesn’t provide kits, so I had to source the components myself. He provides a comprehensive and clear BOM and set of instructions, but it took some patience to find all the necessary components. Jürgen was very helpful in our email exchanges, giving pointers on where to find certain components and what can be substituted.
Tip: if you build it, go to eBay for the HDMI connector. And the B3W-9000 series Omron switches can be substituted with series B3W-9002. The only difference is that the 9002 are stiffer, required a bit more pressure to actuate.
Also his board is a bit more compact than what I am used to working on. You definitely want to use 0.5 mm solder and a fine tip on your iron.
However assembly is generally pain-free since all components are through-hole.
Using
Using the LittleGP-30 is a delight. You only need a USB connection to a computer and a serial terminal program to use it. Jürgen provides configuration files for TeraTerm and I found that Serial on the Mac works very well provided you set the Interpret standalone CR as CRLF option and encoding to ASCII. Minicom works well in Linux, but I have not found a way to handle lone CRs as CRLFs.
When using, you also have to remember that the terminal behaves as if it were an actual teletype. There is no backspace. In fact, there is no number one! Old typewriters and therefore the first teletypes didn’t have a number one on the keyboard and used lower-case l to represent one!
There is also no return key as such. The LGP teletype would have a return/enter key, but that was just for typing. The user had to push the start button on the console to transmit the character to the LGP. Pressing the ESC key on the terminal serves the same function for the LittleGP.
Remember that in the LGP’s day, programs were stored on paper tape, so the terminal program also serves as a virtual paper tape reader. There are digital archives of old paper tapes on line. Following Jürgen’s instructions on how to load a paper tape gives you a very good feeling how using the LGP actually was like. Watching the status lights blink and the LCD/oscilloscope flash with updates as the tape loads is so much fun. Just remember to send the tapes as ASCII, not binary. And don’t forget to hook up a monitor to the HDMI port so you can watch the drum memory fill in real time as you load programs!
Additionally, as a modern and handy shortcut, you can save the entire contents of the LGPs memory as a drum image. This way, you don’t have to wait for the paper tape to load. This isn’t an authentic experience, but sometimes you don’t want to wait to play Lunar Lander.
When sending drum images, send the backspace via the terminal and the LCD/oscilloscope display of the LGP-30 will change the normal drum frequency readout to sho the text “XMODEM” to indicate that it is ready to receive. The backspace will not be echoed to the terminal. Then send the drum image with the xmodem protocol.