Interfacing and testing ROM chips – Part 1


EEPROM chips have come in. (CAT28C256)

Now that we have a low frequency adjustable clock source and a single-stepping instruction interface to the Z80 processor we can test EEPROM chips after they have been programmed. Most every EEPROM chip consists of a certain amount of data and address lines as well as control lines for reading, writing, and enabling the chip. The reading and enable control signals are used by the Z80 for data retrieval from the EEPROM. The enable and writing lines are used to program the EEPROM.

There are many DIY EEPROM programmer circuits and solutions scattered across the internet. There are also many EEPROM programming units that you can buy on the market. These units range from over $500 to under $50. Weighing our options I would suggest saving the headache and buying an EEPROM programmer.

Let me be honest though, even buying a universal EEPROM programmer is a pain. I invested in a G540 universal EEPROM programmer. Its quirks include: complaining about not being plugged in to the USB port when it already is, varying levels of confirmation of success within the software, incorrect instructions for chip placement, and just down-right awful manual instructions. None the less it works for my CAT28C256 (32K X 8-bit) EEPROM chips, and at this point I am thankful for at least getting it to that point. You can get a G540 Universal Programmer for around $30 to $50. If I had a better option for a mid-range EEPROM programmer in the $100 range I probably would have paid the extra money and looked for a more user-friendly software environment.

The next tests I will do will be ROM only instructions. I connect the ROM chip to the Z80, and find some instructions that will only require the ROM. I will use my stepping circuits, and adjustable frequency circuits to make note of the address and data bus.


About Matt Cook

Computer Engineer
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8 Responses to Interfacing and testing ROM chips – Part 1

  1. Jörn says:

    Cool project. Sorry for my bad english. But it is not my native language. I’m still working on a similiar z80 project and using the same book. I used a at28c64 as an eeprom and an arduino and three 74hc595 as an eeprom-burner. Maybe we should come in contact.

  2. drivingdreams says:

    Working on a similar project myself. Hard to find details on the rom monitor. I am planning on using an arduino to write to the EPROM aswell. Is there a file somewhere I can download with his rom monitor, because I want to try and avoid hand programming the whole 1K

    • Matt Cook says:

      Good question. I plan on getting back into the project soon (within the next month) since I am coming off of another, so I will find an answer in the coming weeks and will post more. The monitor program is fairly complex, and I don’t think it will port well to any implementation I will come up with. Its probably best to use it as a reference because it may be more tedious trying to use it as a quick solution. I have found since starting this project that the quick solutions rarely work toward progressing the project further.

      • drivingdreams says:

        Yeah because I have to use my breadboards for that first before I can build any other circuit. So it has been holding me back. I read Steve Ciarcias book and he has lots of good circuits in there. But I need a software version of the monitor. My experience with this type of software is limited so I am definitely stuck

  3. Frédéric says:

    Hi, I’m also doing a Z80 computer and I was curious to see how many people had the same idea as I had, quiet a few lol :D.

    Anyways a while a go I made an EEPROM programmer for arduino and a little C program to send a .bin file through the serial port to the arduino and into the EEPROM. I’ve put all the files and sourcecode in a .rar file in the link below.

    I myself am right now at the painstaking task of putting my breadboard design onto a pcb, after that I’ll try to add UART, but it will be kinda akward since I’ve never used a serial port by the time I started with computers most devices had USB.

    Good luck with your project!

    • Frédéric says:

      I used an arduino mega, lots of I/O pins, so that I could connect all of the EEPROM pins directly, you can get these fairly cheaply on Ebay, and they are pretty handy for other projects to.

      Yes, it should work that way.

      Yea no problem, I gave you the sourcecode (which is actually from the arduino website), in the source code you can easily change the com port, if you don’t know how to code in C++, I can do it for you, just let me know.

      BTW: There’s also a eeprom_read.ino this one doesn’t have an associated exe file, you’ll just have to use the serial monitor of the arduino environment, it doesn’t store the data in a file or anything, it ‘s still pretty useful to verify your EEPROM if the file isn’t to big.

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