A new layout for Orwell.

December 15th, 2013

I decided it was time for Orwell to progress past the breadboard stage. The Usborne reviews are going well. I have added in a silent LOAD mode where character aren’t echoed to the screen which speeds things up a lot. So now the LOAD command is silent and I added a ECHOLOAD command that echoes everything to the screen.

Then I decided Orwell needs a new board. I’ve been drawing out the schematic in Eagle so I can have a PCB made but first I decided to transfer him from breadboard to Veroboard. Not a massive improvement but it meant I could build the board using the schematic as a reference and therefore find any mistakes in it.

Veroboard, or strip board,  is commonly used for prototyping. I use it a lot myself. For simple circuits you use the strips themselves and the odd link across them. Orwell though needs a lot of links so I use a free wiring technique. I put all the components on the board then mark out where the track breaks need to be.

IMG_4546_1

I then drill all the breaks. You can do this by hand but Orwell needed something like 300+ breaks so I set up the pillar drill and used a centre drill to break the tracks. Using the drill I can set the depth and get all the breaks nice.

IMG_4547_1

With the holes drilled I then soldered in all the sockets and components. I added decoupling caps, some of which pass though the track breaks.

IMG_4551_1

I then start adding wires. Now I don’t recommend this technique. I use enamelled copper wire to make all the links (the kind you can burn the enamel off with the soldering iron)  and I use long loops since there are so many connections you will need to move them later to make others. You have to be really careful not to bend the wires so much they break or not to scratch off the insulation on the tops of the pins which will cause shorts. This does happen but I explain how to fix it later. I start wiring on one side of the board, the left, and work across to the right. A little thought is needed to work out which wires to do first so you still have access to the board to do later wires.

IMG_4556_1 IMG_4557_1

IMG_4563_1 IMG_4564_1

I do all the power wires first and then tested the board and measured all the power pins. I then add the rest of the wires. Bloody hundreds of them!

The video board is bolted to the Veroboard and I used mica washers under the nuts to make sure they didn’t short out the tracks.

With all the wiring done (and VERY well checked) I powered up the board. I found there were a few wires I had missed. I would normally print out the schematic and tick off the wires as I add them but my printer died so I couldn’t do this. I also found one short where a wire had lost it’s insulation and pressing on the board caused a short. I figured out which wire it was by prodding about with a wooden toothpick then fixed the insulation with clear nail varnish.

I also made a display board with 8 red LEDs which hang off the debug port and a green LED to indicate power. There was some discussing on whether the debug LEDs should be red of green but as my friend Grant pointed out it’s an 80s computer and KITT was in the 80s and those were red LEDS. I figured yeah, so were Cylons so I went with red too.

It then worked perfectly although I spent a lot of time debugging an issue where the LEDs wouldn’t stay lit. I checked all the hardware many times over before realising it was a software bug. I was using a ROM build that was using the LEDS for other debugging so they were being cleared.

With everything working (I left it running a Larson scanner routine for days) so I started making a base plate for the board so the wires couldn’t move or be damaged. I used some aluminium and made a base plate. The  middle of the sides fold over to grip the board (which tends to bow) and I use small plates on the inside attached by 3mm machine screws to hold the board up.

IMG_4568_1 IMG_4569_1 IMG_4570_1

The board is held up by 12mm standoffs I machined up from old chopping boards. The middle supports hold the board steady when plugging in the cables.

So now Orwell has a nice, slightly better than breadboard, circuit board and he is working well. Much neater on my computer desk too.

IMG_4571_1 IMG_4573_1 IMG_4577_1

So next would be a proper board which I will work on but for now Orwell is working well.

And I just found one of the hard to find Usborne books on eBay so soon I hope to have a whole set so I can complete all my reviews.

 

 

Comments are closed.