Initial Testing

A project log for $5 Graphics Card For Homebrew Z80

Using a Pi Zero, I want to create a cheap keyboard interface and graphics card for my RC2014 homebrew computer

spencerspencer 02/07/2016 at 19:110 Comments

After getting my paws on a Pi Zero, I needed to check three important things; Can the serial port talk to the RC2014, how do I use the composite output and what terminal emulation program does the NOOB install contain?

I don't have a HDMI monitor, so needed to use the composite output of the Pi. Soldering a photo adapter on to the output pins was easy enough, but initially, I couldn't get it displaying anything. Eventually I realised that you need to hold down 3 on the keyboard when it started up.

Once I had the Pi up and running, I quickly came to the conclusion that there weren't any terminal emulators included by default. So, I would have to download one... which meant I had to find a USB wifi dongle. And a hub. Ok, so after some trial and error,

sudo apt-get install screen

got me up and running with a command line terminal program. Next, to sort out the serial port stuff.

The Pi runs at 3v3 and the RC2014 runs at 5v, so plugging the two together isn't quite so straight forward. I know there are level converters that can allow 5v devices to interface with 3v3, but I wanted to try keeping things simple so used a 10k and 22k resistor as a voltage divider on the TX pin from the RC2014 to Rx (pin 10) on the Pi. This works out to 3.44v, and although the Pi isn't 5v tolerant, 3.44v seems to be close enough.

I ran the TX from the Pi (pin 8) directly in to the RX of the RC2014. Although it isn't the 5v isn't as high as the RC2014 would like, 3v3 is sufficiently high that it works ok. And, of course, it needs a common ground.

Initially, starting up Screen;

screen /dev/ttyAMA0 115200
didn't work. The serial port is counterintuitive in that it needs to be turned off, either via the GUI settings or editing the cmdline.txt file, in order to use the serial port. Turning off the serial port means the Pi doesn't use it for it's own stuff, so the user is free to make use of it. Firing up Screen now works and the Pi behaves as a graphics card for the RC2014!

Next up, making a dedicated PCB for easy connection to the RC2014 and automating the boot process...