Easy add-on Projects for Spectrum ZX81 & ACE (Redux): Part 2

Building the Decoder Board

After several weeks of waiting the Decoder Board PCBs arrived just in time for a bit weekend assembly and testing. I'd ordered the other components required for the build a couple of weeks ago and had them laying around in anticipation.

Aside from the various electronics, I decided to use a ZX81XT extender board purchased some time ago from Sell My Retro. The ZX81XT is perfect for prototyping or just for swapping various project boards in and out without the need for multiple edge connectors.

ZX81XT extender and Easy Add-On Decoder Board PCBs

As my decoder board at its core is a part for part update on the original design as presented in the 'Easy add-on Projects for Spectrum ZX81 & ACE' book, I pretty much followed the general assembly instructions given in the text. This process was exceedingly helpful during the various testing phases outlined during the build procedure.

The major update in my version of the decoder board is the substitution of through hole components for SMD parts. Being the first time I've willingly used SMD parts I was expecting quite a challenge. The good news for anybody else finding themselves slightly apprehensive of incorporating SMD components into projects is that it's actually quite easy to use them. The most problematic parts I found to be the tiny resistors and capacitors. I managed to loose a couple of the minuscule resistors by simply dropping them into someplace never to be found again. Dealing with and soldering of the SMD ICs in comparison was all very straight forward.

Fully Built Decoder Board Ready for Testing

After about an hour and a half I had the decoder assembled ready for some final testing. This is where I hit a slight snag. The first project in the book, 'Pulse Detector' serves as the final assembly test for the board. Unfortunately I neglected to order the ICs required for the test project.

I figured it should be possible to conduct some testing with a simple 555 timer circuit. I did however wish to ensure that what ever it was I built would be genuine test of the decoder. Luckily the book had such a 555 timer circuit I could easily modify, 'Project 5, Beeper'. Simply removing the Beeper part of the circuit (which I also had no components for) and replacing it with an LED would prove the board worked.

LED 555 Test Circuit Connected and Working on a Spectrum Omni.

I first tested the Decoder Board on a ZX81. This requires entering a program listing appearing in the books appendix. The ZX81 has no built in BASIC commands to read or write to expansion BUS, the programs listed in the book addressed this absence. After entering an running the application I successfully blinked an LED, a positive right of passage for any micro controlled project. I then moved the decoder board across to my recently acquired ZX Spectrum Omni (deserving of it's own blog entry sometime soon). Again the LED blinked as expected.

All in all everything has worked as expected, time to pick up some extra electronics and build some of the books proper projects.

See more entries for this project: Forward, Part 1, Part 2, Part 3

Easy add-on Projects for Spectrum ZX81 & ACE (Redux): Part 1

Re-Design the ZX81 Decoder Board

In the previous blog entry I left off before starting design work on the Decoder Board. The board is an essential build before attempting any of the 'Easy add-on Projects for Spectrum ZX81 & ACE'. In reality it's the first project of the book, even if it's not fully described until Appendix A.

First up I needed to transfer the circuit diagram provided in the book into Fritzing and build up a new circuit board compatible with the ZX81. I really could have made life easier for myself simply by printing the circuit board designs included within the book. My plan however is to use surface mounted ICs' instead of through hole components, both modernising the design and shrinking the boards footprint.

Decoder Circuit as presented in Easy add-on Project for Spectrum ZX81 & ACE

My intention was also to follow the original Decoder Boards layout reasonably closely. Ensuring any projects described by the book would still be referential to the end product. My main concession was to design around the shorter ZX81 edge connector only. This does not prevent a ZX Spectrum from using the device as a Spectrums' BUS is compatible with all signals used by the board.

To make life somewhat easier I combined the top and bottom layers of the circuit board layout presented in 'Easy add-on Projects' pages 166-167 into one image. Using the composite picture as a guide I began the layout my board. I'm quite glad I took this route as I noticed a couple of minor errors I'd made in transcribing the circuit diagram. More interestingly however the described circuit didn't seem to match with the board layouts provided by the book.

Composite Image of top and bottom board layouts as taken from the book.

A couple of traces from IC2 to IC4 appeared to be different between the books board layouts and circuit diagram. This was more than a little confusing. I checked the Jupiter Ace board layout (also in the book) against the ZX81 / Spectrum boards, and the designs seemed consistent. I decided to go about about updating my circuit diagram, bring it in line with the book layouts.

After quite a bit of triple checking I now have a circuit board design ready to send off for fabrication. My new (mostly) SMD board is approximately half the size of the original design, good for keeping fabrication costs down. I've also added a few extra clarifications and helpful labelling to the silkscreen layers.

Now to order some parts, check the design again before sending it off and hope for the best.

Final design for the new Decoder Board

See more entries for this project: Forward, Part 1, Part 2, Part 3