Build Process (Still debugging)

[marcwolf60]

My Homebrew PDP8/I

I'll have to pick up a 5.5v plugpack as the supply I am using is only a 5V 3A one.  Also as the end result will be running on a Pi Zero W I need a mini HDMI to HDMI to view the screen.

I used the Gerber files and got AllPCB.Com to make the board for me. The minimum quantity they do is 5 but that still came to about $45 drilled and silk screened.  I was very impressed by the quality. I ended up with 8 boards and have given several to friends and pointed them to your site.

I picked up 100 water clear yellow LED's  on eBay for $10, and the rest of the parts I had in my 'bits box' as I do a lot of electronics for fun. After placing them in to the board correctly I then put a section on plywood on top of the leds.. flipped the board, and then soldered in place. After cutting the leads I did a second pass of heading the joints with the soldering iron and wiggling the LED to make sure it was firmly seated and aligned with the others.

I had 26 IN4004 diode that I used - bending the leads a little more to make then fit.

The front panel was made by using your art work in Gimp 2, setting the transparent layer to white and sending it to the back. I printed on A3 paper and then laminated. The light section is nicely grayed out and shows the LED's shining nicely.

I used standard  ON/OFF SPST, and momentary ON/OFF/ON SPST (with one lead removes) switches. I needed to bend the leads out a little to make them fit and held them in position but soldering the outside pin first. Then I could 'wiggle' the switches until they were aligned and solder the other pin. Then like with the LED's I did a final heat and wiggle for a final alignment.

For the handles - I am not keen on the total authenticity of the PDP8/I's switches but prefer the PDP8/e ones. I will draw up a design using Fusion 360 and then 3D print in white PLA at 0.1mm layer to give a nice smooth finish. They will have a hole for the switch shaft to slide in to the paddle part and held with a spot of  hot melt glue. Since the switch shafts can rotate thus cause 'wobble' I will also put a piece of felt between the switch paddle bases to stop that.


My background is that is an IT manager/Electronics Tech with a keen interests in animatronics. I cut my programming teeth in the early 70's on the Schools PDP 11/40. I'm building the PDP8/I because I liked seeing the actual instructions being processed. I had great hoped to build an EDUC-8 when  it came out but alas pocket money would not afford it.   https://en.wikipedia.org/wiki/EDUC-8
I also have a Mini-Scamp that I am slowly getting working again  http://www.oldcomputermuseum.com/mini_scamp.html
Also I would sneak (at 15) down to the local University where I befriended a student making a Signetic's 2650 board. I drilled my own board and  hand soldered a  1K of 2012 static ram.

Enjoy the build photo's. 

Future developments... I am looking to see if there is a spare GPIO pint that I can use as an A/D.  I will attach a potentiometer  to it and mod the SIMH program to read it's value and use it as a 'slowing loop' so I can vary the PDP8/I's running speed

Dave.. also known as Marcwold on various boards anf forums.

[Gordon Henderson]

On Sat, 10 Mar 2018, marcwolf60 wrote:

> I also have a Mini-Scamp that I am slowly getting working again
> http://www.oldcomputermuseum.com/mini_scamp.html

Hm. Must have missed those - I have a working Mk14:

https://unicorn.drogon.net/stuff/mk14-e.jpg

same SC/MP processor..

> Future developments... I am looking to see if there is a spare GPIO pint
> that I can use as an A/D. I will attach a potentiometer to it and mod the
> SIMH program to read it's value and use it as a 'slowing loop' so I can
> vary the PDP8/I's running speed

The Pi has no native analog inputs, but you can use the I2C bus on the Pi
(assuming you didn't do the mod to keep the on-board serial port) and use
one of many I2C ADC chips, or you can use the single pin left as a 1-bit
ADC via a resistor/capacitor switch - you'll need some code that sits in a
loop alternatively setting the pin to output mode, 0v to discharge the
cap, then input mode and then time how long to read a logic 1. I've done
this to read a simple ORP12 LDR in the past. I'd suggest a 100Ω resistor
on the pin input to limit inrush current when you discharge the cap ...

Cheers,

Gordon

[William Cattey]

If you use the "8e style" paddle switches, you should change your artwork to call it

a PDP-8/L which actually did have that style of switch.

[Oscar Vermeulen]

And check out so-much-stuff.com, the CAD section. Vince Slyngstad made 3D printable paddles in the 8/e style, which have reached a very high degree of perfection and fit normal C&K switches.