Why 1:2 Scale?

I decided that I wanted to match the scale of the PiDP-1 as close as I could. But what was that scale? I did a little investigating.

According to the Computer History Museum the width of the PDP-1 rack was about 27 inches. So I brought this photo into Autodesk Fusion:and "calibrated" the image to 27 inches (top blue line). Then I measured the width of the console (bottom blue line) which was 20 inches. I had asked on the PiDP-1 Google Group for someone to send me the measurement of the PiDP-1 Console at it's widest point. 

[rbclar...]  informed me it was 11 inches. So I calculated that the PiDP-1 is about 55% the size of the original PDP-1. Cool. 

Then Oscar Vermeulen added the following to the thread:

Here is the most important  measurement I got from Bob Rosembloom @CHM.  (PDF attached). (Caveat, there might still be some errors in this PDF, there was a long an winding process. But I think this was the final version)
The line you highlight is 19875 mils (504.825mm) on the real PDP-1. It's 283.21mm in the PiDP-1. ...crunching... the exact scale is then 1:0.561

...or about 56%.  Nice to have confirmation. Thanks Oscar.

With respect to wanting to make a smaller Type 30 display it turns out that a 55-56% scale works out well for using the Pimoroni PIM-372 8" 4:3 Display which I had used in other projects.  It is even a better fit at a 50% scale and since a scale of 1:2 is much easier to say, remember, and visualize than an 11:20 scale I decided to go with half scale as being close enough.

A Fresh Start

I was tempted to just scale my existing design down to 50% but worried that saving a bunch of time up front might lead to a lot of unforeseen issues on the back end. I decided to start from scratch. Besides I'm retired and have the time ;-)  Also in my effort to start learning how to program a PDP-1, I had been reading "Retrochallenge 2016/10: Ironic Computer Space Simulator (ICSS)" a wonderful recanting of the effort it took to reproduce an arcade video game from 1971, Computer Space on the DEC PDP-1. There I came across the following image:

with the caption: The Ironic Computer Space Simulator running on the emulated PDP-1. 

Based on this, I think what I'm seeing is a photo that has been brought in as a background to the game screen over which controls (upper right) and the game display are being superimposed. To me this is the most perfect "front on" image of the Type 30 (?) Display that I had ever seen. Every other image I had seen is shot at an angle (sometimes small) that introduces parallax effects when trying to make measurements. One other caveat is that there is no guarantee that this is a Type 30 Display image.  I never-the-less decided to use this image as the basis for my smaller model. 

So I brought this image into Fusion, "calibrated" it based on the screen cutout being 8 inches (half the originals 16 inches - see how easy that was ;-) and proceeded from there to model the rest using the now calibrated image as a guide. 

Benefits

There are a number of benefits with switching to a 1:2 scale reproduction:

• As mentioned reclaiming desk space.
• Better match to the PiDP-1 as can be seen in the top picture gallery.
• The 8 inch Pimoroni display was about $100 cheaper than the 10 inch and appears to be more readily available.
• Most of the parts can now be printed on an A1 Mini making the model more accessible.

Building One

You will find a list of the required parts, all of the necessary build files, and assembly instructions in this project needed to make your own 1:2 scale DEC Precision CRT Display Type 30 Reproduction.