Wednesday 26 April 2017

ZX Spectrum Project - Part 2

Today on the Spectrum I removed all of the old lower RAM chips and added sockets. I have spent the day trying to get a modern SRAM chip to take the place of all eight of  the DRAM chips, timings and support logic are proving to be a bit more of a challenge than I had anticipated. This has resulted in some extremely weird graphics on the screen. I have found that adding a little bit of logic can produce some unusual graphics.

Next I believe I am going to try using a micro controller to better manage the conversion as it appears that the signals have a bit more of a timing requirement rather than being purely logic driven. Think the ATTiny2313 for this task as it doesn't need too many pins and I don't think there will be too much processing power required.

I have also been working on the keyboard controller. This is purely theoretical at the moment as the Spectrum is doing the weird graphics stuff but this appears to be decoding the keyboard signals and mapping them to the correct responses.

I think tomorrow I will be attaching that to my other Spectrum to test it out.

Tuesday 25 April 2017

ZX Spectrum Project - Part 1

The project has started off quite simple. I removed the case and the RF modulator and gave myself a composite output. I then removed the 7805 and the coil and connected a USB-B socket to the board to take a 5V input. I am now realising that the graphics memory requires 12V and -5V to power it as all I am now getting on the screen is the flashing bars of doom. I am going to try and replace this memory with a more modern device, preferably a single chip rather than eight.


I am also thinking about what I want to achieve from the board. I am definitely going to be attempting to recreate the Spectrums ULA using a combination of some logic and a microcontroller, this will be challenging but there is enough information about the part out there. I will probably also create a new PCB as the old one uses a lot of one bit memory chips which eat up space where the modern eight bit parts will be much smaller. I think the new PCB will be more than achievable on a dual layer 100mm X 100mm board. I also think that it would be a good idea to integrate two game ports rather than having an edge connector on the back. Also, being able to use a modern keyboard would be a plus as the Spectrums keyboard is horrific.

As I have no plans to use the original keyboard I have removed the connectors and replaced them with pin header which is much easier to connect to.

Parts used so far
- 1 x ZX Spectrum with issue 4b board from 1983
- 1 x USB-B connector
- 1 x Phono RCA connector
- Wire

Pictures from today