Monday, June 26, 2017

PiDP-8 USB Hack

I have just finished building a PiDP-8/I from a fantastic kit offered by Oscar (Obsolescence Guaranteed) Vermeulen. This is a 2/3 scale reproduction of the front panel of the Digital Equipment Corporation PDP-8/I computer. Inside is a Raspberry Pi using SIMH (See also: SIMH project on GitHub.) to emulate a PDP-8. The PDP-8/I has a special place in my heart as the first computer I really got to know. You always remember your first...

One of the challenges is to bring the various and sundry connections through the bamboo case out to the world in a pleasing manner. I managed this:

Details on the connectors:

The power, audio, and HDMI extenders were findable with care.

Power was pretty straightforward.  I went with one from Renrii sold through Amazon, but I see it is no longer available, only a month later. Word of advice, look closely at the pictures, "Left Angle" and "Right Angle" are used inconsistently from vendor to vendor.

Audio extender to the box was straightforward.  I picked up this one from Amazon:  [Benly-38] Right Angle 4-pole 1/8" 3.5mm Stereo Plug / Male to Straight 4-pole 1/8" 3.5mm Stereo Jack / Female Aux Headphone Cable 0.55 Feet (17cm) Wvyr-4l4sf-15

But once outside the box, if you want to split the 4 pole 1/8" jack into separate RCA connectors, you must buy VERY carefully.  The assignments are not standardized, and some of the splitters have ground where you need signal.  My recommendation:  Buy new/old stock Zune a/v output cable. I got mine from a third party seller on Amazon:  Zune A/V Output Cable (Discontinued by Manufacturer). I confess I've not tested this yet, so I'm not 100% sure this is right.

For HDMI, I tried a cheaper one, but it failed to fit.  So I went with the Vanco 233306X 6" Right Angle Super Flex HDMI High Speed Male to Female Cable with Flat Top from Amazon.

I used standoffs to mount the PiDP-8/I, and then glued a 1/4 inch wood shim to the bottom of the box. This is to improve friction fit, serve as strain relief, and provide a surface to epoxy to.

I did some careful measuring and established a baseline for my connectors that would descend a bit into the wood shim.  Then I drilled a couple holes, and used a rotary tool and wood carver's tools to make the holes mostly shaped like the connectors.  A couple times, I lost control of the rotary tool and scraped up the box.  For example the line above the power connector you see in the final picture below. My holes are less than perfectly shaped.

The USB Hack:

The USB and hard-wired network connectors of the Raspberry Pie were quite close to the edge. Some people just cut a hole in the box, but I wanted to try and make an extender that would work in a confined space. But there is very little clearance between the Raspberry Pie and the case.

For me this meant cutting up a standard USB connector and hacking up a cable. My first attempt failed as the pins broke off. Here are the steps I took to get to the cut-down connector that did work.
Mouser Electronics was a quite reasonably priced source of male and female USB connectors.

1. Start with a USB Male connector.
2. Use a cutting wheel.
3. Cut the sides at the notch. 

4. Side cuts to guide the main cut.

5. Cut the metal.

6. Then the nylon 

7. Solder it up and your done?

8. The pins break right off.

9. Second try. Cut here.

10. Clear the nylon above the pins. 

11. The wires fit well in the well.

12. Power, Signal, and Chassis Ground.

13. Magic elixir!

14 Protects physically and electrically.

15. Carefully carve out  the case
where the USB connector will land.
16. NOW you are done!

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