RC2024 – Part 12 – Building And Testing The Updated ROM Board

ROM Module for the RC2014 with ZIF socket and microswitches

Earlier on in the Retro Challenge I wrote about designing a new ROM board for my RC2014. This was going to be higher than the standard narrow board for the RC2014 Classic 2 computer. It was also going to have switches on it to swap the memory map around, and a ZIF socket to allow me to program new ROMs.

The updated PCB arrived from JLCPCB, so I attempted to solder it up. I made the same mistake as on my Rotary Encoder Module PCB, and left the spokes to the ground plane too large. This made it difficult to solder the ground pins as the heat from my soldering iron was being wicked away. It took some time, but I was eventually able to solder those pins.

I removed the old ROM module and swapped the ROM chip over into my new board. I turned on and… nothing. I checked my soldering and it all looked OK. I then touched the ROM chip, ouch! That super hot. What had a done wrong? I went back to check my circuit diagram and that all looked right. The traces were going to the right places. Then I realised what I had done. The comment I’d put on the board to remind me which way round the ROM chip went as pointing in the wrong direction. Reversing the chip brought my RC2014 back to life. Thankfully I hadn’t destroyed the chip.

By default the board booted into Microsoft BASIC. So I switched it off, and flipped the switches to all be on. Turning the power back on, I was greeted by the SCM (Small Computer Monitor). The board was working as expected! I could now easily swap between BASIC and SCM without removing the card and having to swap jumpers on the board.

Using My Own ROM

Apart from being able to swap between BASIC and SCM, I wanted to be able to use my own ROM chip.

I have some Winbond 27C512 EEPROM chips. These are pin compatible with PROM chip that Spencer supplies with the RC2014 Classic 2, but are reprogrammable.

I have a Xgecu T48 programmer for programming chips, but I could not get this to work on Windows 11 using Parallels on my Mac. Although it sees it as a USB device, the programming software would not recognise it. I needed a Mac solution.

For this I found minipro. It’s command line only on the Mac, but it works with the Xgecu T48 programmer. It can easily be installed using Homebrew

brew install minipro

Spencer has the RC2014 ROMs available as binary downloads on Github, so I found the one compatible with the RC2014 Classic 2 and downloaded it. The image I used was R000009.BIN.

To burn it to the EEPROM, I had to use the following command.

minipro -p "w27c512@dip28" -w ~/Downloads/R0000009.BIN

This tells minipro I’m using a Winbond 27C512 EEPROM, and I want to write the file R000009.BIN to it.

Once programmed, I could verify the contents using the following command.

minipro -p "w27c512@dip28" -m ~/Downloads/R0000009.BIN

I was able to easily swap the chips over in my new ROM board thanks to the ZIF socket. I turned on the RC2014, and I was greeted with the SCM prompt.

I now have the ability to program my own ROM chips and use them on my RC2014 Classic 2.

Next Steps

I have tweaked the PCB layout so the spoke to the ground plane are a more sensible size. More importantly, I have corrected the text showing which way round the ROM chip should be placed in the ZIF socket. While I was tweaking the layout, I decided to rotate the 74HCT32 chip so it had the same orientation as the as the ROM chip. I also moved the decoupling capacitors to the left hand side so they are next to the the 5V pins.

This has been sent to JLCPCB for manufacture. As the current board works, I have used their cheapest and slowest service. The new board should be with me in a few weeks.