Tag: ROM

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

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.

RC2024 – Part 4 – A Quick Diversion To Look At The RC2014 ROM Board

I have been thinking about what else I want to achieve with this year’s Retro Challenge.

Apart from designing a PCB to interface rotary encoders to my RC2014 computer, I wanted to be able to use them from software. I have used BASIC for my testing so far, but I also want to look at using Z80 assembly language.

To use Z80 assembly language on my RC2014 Classic 2, I have a few options.

  1. Use BASIC to load and execute a hex dump of the my assembled Z80 code.
  2. Use SCM to load and execute a hex dump of my assembled Z80 code.
  3. Burn my assembled code into a ROM and run that directly on the RC2014.

The ROM board for the RC2014 Classic 2 has the ROM chip socketed. If I wanted to use my own ROM image I can lift this out and replace it. However, the board is low down and I would need to also remove the board from the RC2014 backplane. This would add mechanical wear and tear.

I have a similar problem if I want to switch between ROM images. For example, from BASIC to SCM, or to my own custom ROM image. There are jumpers on the ROM board, but I would need to lift the board from the backplane to be able to swap them.

So, what is the solution?

Well, Spencer does have an RC2014 riser card available on the Z80 Kits website to lift the board above the other cards. This makes it possible to reach the jumpers, but still difficult to remove and replace the ROM.

After writing about designing the PCB for the rotary encoder, I thought I could just build my own ROM card. What I want my new ROM board to achieve is…

  • Use a ZIF (zero insertion force) socket for the ROM so I can easily remove and replace it.
  • Use switches instead of jumpers for the ROM select so I can easily change which part of the ROM the RC2014 is seeing.
  • Have the ZIF and switches above the other boards so I can easily access them.

The circuit design of the ROM board is essentially the same as the original ROM board, just with some pull down resistors near the address switches. The board height will be the same as a standard RC2014 board.

I couldn’t find a DIP package with 3 switches in the EasyEDA library, so I have used a 6 pin socket in the design which should have the same footprint.

Here is my PCB layout.

I have sent this off to JLCPCB to be manufactured. The cost including delivery was just £2.53 using their 10 to 14 day delivery option. I’m not in a rush for this board, so I think that is a fantastic price. There is also a $2 discount at the moment, so that also reduced the cost.

I am expecting the board to arrive towards the end of the month but this won’t stop work elsewhere in the project.