An easy to build programmer for 2704 / 2708 / MCM68764 / MCM68766 EPROMs

Introduction

Some time ago I needed to program an MCM68766 EPROM to upgrade the BIOS in an IBM PC 5150, and in the process discovered that hardware which can program these is rather difficult to come by. Despite having a collection EPROM programmers, including one fairly expensive model (which apparently could, but wouldn’t do it in practice) – in the end none were up to it.

So I set out to build my own. Knowing that the pin-out for 2708 EPROMs was quite similar to MCM68766 – I thought I’d tick that off while I was at it, as this is another type that is very difficult to find hardware to program.

The original prototype – designed late 2016 and mentioned on vcfed.org forums early 2017

Why is MCM68766 difficult to program?

There is nothing algorithmically complicated about programming one of these – the main difficulty centres on the high Vpp voltage (26V). Most cheap EPROM programms have a max Vpp of 13V. That having been said, there are some cheap programmers which support the 2716 EPROM, also needing a Vpp of 26V, so quite why MCM68766 is such a difficulty, may also depend on other things, such as pin arrangements, or perhaps just limited demand for it.

Why is 2708/2704 difficult to program?

This type of EPROM is genuinely more difficult to deal with. It also needs the same high Vpp voltage, as well as an additional cocktail of voltages. +26V, +12V, +5V and -5V being the full list. All of these are required during programming – quite a headache for the designers of universal programmers. Models that do support it, require a special adapter which doesn’t come cheap. This adapter typically supplies the extra voltages which aren’t practical to provide through the universal socket. Like the MCM68766 – they are algorithmically simple to program, so no complicated software stuff to worry about.

Design

The physical design is in the form of an Arduino Mega shield. I chose this because it was large enough to accommodate all of the components, and there would be enough I/O pins without needing I/O expanders or other complicated stuff.

The only supported Arduino is the Mega 2560 R3 or compatible.

When designing it I was aware that I was likely the only person that’d want something that did both, so have allowed for it to be constructed as a dual 270x/MCM6876x programmer, or 270x only / MCM6876x only.

When built in its dual type form, selection between devices is made by a 6PDT switch, which software can query before powering on the device, checking against the selected device in the user interface, limiting the chance of accidentally blowing an EPROM (which will certainly happen if the switch is in the wrong position).

When programming 270x EPROMs, the Arduino must be supplied with a regulated +12V supply as the shield does not generate its own +12V rail. There is a software check to ensure this voltage is correct before powering on the EPROM.

There is also a footprint for an RS-232 connection + MAX232 but this is not required for most applications as the Arduino can provide a serial interface through its USB port.

Constructing it as a dual type programmer

All components must be fitted except for the RS-232 section.

Constructing it as a MCM68764/MCM68766 programmer

I figured it would be easier to just show a picture of this. All fitted components are as specified on the schematic, unless omitted or replaced with a wire link.

SW1 is hard wired in the left hand position.

Constructing it as a 2704/2708 programmer

When constructing as a 2708 programmer, only the following components can be omitted: SW1, J1, U4, D2, C1, C8, C9, C10, C11, R9, R16.

SW1 must be hard wired in the right hand position.

Buying one pre-built

I don’t sell these things myself. Search for “2708 programmer” on eBay. I’ve seen a number of examples of people selling my design pre-built. There is now at least one “compatible” design out there – different hardware, but using my software.

Host software / firmware

Please see the main project page for more information.

Schematic

Can be downloaded here.

Gerbers

There are two versions of the gerbers. A “Long” version which has the exact dimensions of an Arduino Mega. Choose this if your PCB house doesn’t charge extra for exceeding 100x100mm dimensions. Total size is 101.5mm x 53.5mm.

The “Short” version has the tab at the end clipped to keep it under 100mm. Total size is 99.95mm x 53.5mm.

BOM

Can be downloaded from here (or here – CSV).

2708 Voltage range warning

When developing this shield I found that some 2708 devices are fussy about the voltage on the +12V rail – they won’t be damaged but they’ll not program successfully. The designers of the Arduino Mega have helpfully fitted a reverse protection diode (D1).

While this does prevent hapless newbies from blowing their Arduinos, unfortunately in our case it reduces the 12V supply down to 11.2V. For reliable 2708 programming either this diode must be replaced with a wire link, or the Arduino should be powered with a 13V supply.

54 thoughts on “An easy to build programmer for 2704 / 2708 / MCM68764 / MCM68766 EPROMs

  1. Hello,

    I hope you don’t mind, but I am going to have some of your shields made up by Oshpark for myself and a couple of friends who are involved in the repair of Vintage computers.
    I have sourced some MCM68764/66 EPROMs and this will be great for replacing or upgrading old 24-pin DIP ROMs.
    If it’s OK, I’m going to share the ability to have the boards ordered from Oshpark as needed.
    No one, other than them, will make money off this and I will have the board design credited to you.

    Thank you for your work in designing this shield.

    Regards,
    Richard

  2. I built your programmer and everything worked on the first try!
    I’m really excited about having programmed an EPROM 2708 for my old computer.
    Thank you so much, you were incredible, very good … and sorry for my bad English 🙂
    Greetings from Italy

  3. Built today your 2708 programmer, worked from the first power on, great project, thank you for sharing. Was curious, because I used a 12V Zener instead of a 11V and 220uH inductors instead of a 180uH, but the voltage values were ok, and I was able to program a 2708 EPROM. Are I on a “dangerous” way with this componets?

    1. The changes you’ve made wouldn’t make a lick of difference to operation. The zener is to keep Vgs below 20V but still allow fast gate charge time, so 12V will still be OK.

      As for the inductor, also not going to be an issue. Going higher can result in higher losses (+overheating) and slower regulation response time, but at 220uH it’s still going to be well within operating margins.

  4. Thanks for the effort.
    I wanted to display the checksum display automatically when loading the binary file or reading out the EPROM.
    Couldn’t you get the source code after all?

    1. Perhaps you could look at writing a specific programming tool which does exactly what you want? I’ll never be able to please everyone! I’d certainly be happy to share it here. Happy to provide any protocol information needed.

  5. Would be nice if you shared the sources, so that you can customize the software according to your needs.
    Kind regards

  6. I wanted to make the software a little more comfortable.
    For example, immediate CRC display in the user interface, use as a pure 2708 programmer and possibly an editor.
    I did not want to expect all of this from you.
    Kind regards

    1. If you are looking to create a highly customised interface – I’d ask that you build it from scratch rather than customising mine. If you are in a position to integrate a binary editor – this should be no difficulty for you.

      As I’ve previously said, I do not intend to release the source for the Windows UI as I don’t want bastardised versions of it all over the place, which I then have to support.

      I have added the source for the command line version which should be a very good starting point:

      https://github.com/inaxeon/hvepromcmd

  7. I last programmed a 2708 with a borrowed burner for my Commodore 64 club. 1990 or so. Always wanted to play with them since.
    I’ve been mostly wanting to burn spare chips for older test equipment and other stuff.

    Ideally I would like to just read older chips and burn to some pin-compatible new stuff. I assume eeproms for 2708 and newer are available.

    I have unused burners somewhere, but was going to order a modern burner like the TL866.

    Also for my Heathkit HERO 1 robots and et-3400a microprocessor trainer and peripheral adapter roms.

  8. Hello Matt,

    I am maintaining old school arcade game pcb. As you know, it is difficult to use programmers who read and write old eproms now.

    I’d like to use your 2708 eprom programmer. I know you don’t sell it yourself, but it’s very difficult because there’s no place to buy it. I know one seller on eBay, but I can’t make a purchase because of the delivery problem. He said that shipping from Germany to Korea is difficult due to costs and customs clearance.

    I would appreciate it if you could sell the finished product or kit for me. Prefer finished products to minimize errors in production. I will pay for the cost and delivery charge by Paypal.

    I live in South Korea. I look forward to your positive answer.

    Thank you.

    1. Hi,

      is anybody ordering/making these pcb’s?
      i am interested to have one…

      struggling with my 2708 here

      thanks,

      Wim

  9. Hi Matt,
    congratulations to your outstanding projekt.
    I built a programmer for my Arcade repairs and it worked from the first try!

    Thank you very much for sharing your project!!!

  10. Hi Matt,

    I built the 2708 version of your programmer. I’m getting “Chip is not blank at 0x0000, Data 0x00” on what I believe is a blank chip. If I save to a .bin file and open in a hex editor, I’m seeing 00 in every position. All the voltages looked correct when I ran the self test.

    Any guesses what could be wrong here?

    thanks
    Charlie

      1. Hmm, the test looked good last night, but now I’m seeing wrong voltages in the tests. I do have 12v coming from my power supply.

        5v is reading 4.2v, -5v reading .3v, 12v reading 1v, 4.2v reading 1.8v. Sounds like I managed to burn something out!

        Also, no more buzz sound on the 25v step. This looks to be pulsing between 0v and ~10v, but it is difficult to tell the exact high number

        I had not tried the read test yet, (I’m assuming that is the final one with the resistors).

        I’m going to go back to my bench to inspect my soldering job for any mistakes and also double check that I didn’t put something in the wrong spot.

        Anything jump out at you as possible failed components that could do this?

  11. My minipro programmer has a max VPP of 21V, this is not enough for some of the 2716 eprom I have. I have built this programmer (thanks for sharing you design by the way) and am thinking it should be able to support these eproms too. Is it possible to add support for the 2716?

    1. I was looking at this recently. Unfortunately it’s not just a software tweak, significant change to the hardware would be required.

      1. Yes, I was just looking at the pinouts and realised that VPP /CE and A10 would need to be switched around. Maybe it’s easier to design an adapter board that switches these pins to the right place? Still would need some adjustments to the software, or have a jumper tie A10 to gnd or vcc and program both halves separately.

        1. Software changes not an issue, having an adapter would be a bit of a pain. It may make more sense to revise the board so that the switch changes between 2708/2716(32) and support MCM6876X with an adapter, given that few build it for this. But, considering that 2716 capable hardware can be had for $30 – I don’t think many would bother.

        2. Interestingly the 2732 can be supported with a software change. Maybe it might be worth knocking up a small adapter for 2716’s…

          1. If the 2732 can be done with only software, that would be great. Hoping you can add this feature and maybe even the 2717 with adapter board.

          2. I may be wrong about that. The sticking point is that all 2732’s that I could see are 21v Vpp, so there’s be a small hardware change to allow software selection of the programming voltage.

            Probably one for a future revision of the shield I think…

          3. Actually I have many 2732’s here that are 25v Vpp so I am hoping for support for these. The 21V Vpp 2732’s I can program with my TL866.

  12. Update on my issues above. It turned out the MC34063 was failed. I replaced it and now my voltages are correct.

    Unfortunately I am still getting “Chip is not blank at 0x0000, Data 0x00”. The resulting bin file is all 0s which I think means blank. The hardware tests all pass with the correct voltages and the read test with resistors works correctly for all the inputs.

    Any suggestions on what else to check?

    PS – Here’s a link to the bin file if that is of interest

    https://1drv.ms/u/s!AqiyUGOT8PbNw-tEeGj1u-EIkMvWZQ?e=ODDmBn

  13. I just have one 2708. I will trying ordering some others in case this one is bad.

    I figured I have nothing to lose with trying to program this 2708, so I just turned off the blank check option and tried programming it anyway. It made a few passes and U1 exploded in a puff of smoke and sparks! Below is a pic of the U1 aftermath…

    I have U1 socketed and was able to replace it quickly. The next time I tried it was able to make all 100 passes but it doesn’t look like anything was written. I will try with a different 2708 and see if that helps

    U1 explosion!
    https://1drv.ms/u/s!AqiyUGOT8PbNw-tJrwbP2DYdOMMLjQ?e=XrR7YL

  14. When burning a TMS2708JL Eprom, an error message occurs.
    The comparison immediately after the burn process reports “Verify error”.
    But if Verify is called individually after burning, it is correct.
    Can you insert a pause before the automatic verify in the software?

  15. Hi Matt – You were right. I had a bad 2708. Replacement 2708s seem to work fine.

    One issue I noticed is blank / unused memory areas of the programmed chip have “FF” in them. I see this if I write a .bin file that is smaller than the total size of the 2708. This leads to a different checksum when comparing the chip contents to the original bin.

    For example, if I write this 2 line source file, (rom #9 from the arcade game, Magical Spot)

    https://1drv.ms/u/s!AqiyUGOT8PbNw-tzHjdzqXG2hdPQHA?e=cNS9C9

    it seems to write fine, but I then get this slightly different file when reading the 2708 and saving as a bin.

    https://1drv.ms/u/s!AqiyUGOT8PbNw-t0PJZXLhlLBvZZfg?e=RmrjwU

    If I view the original file in a hex editor, there are just 2 lines of code. The saved bin has those 2 lines, followed by many lines of “FF”.

    Is there any way to eliminate all the “FF”s in this situation so the files are identical?

        1. Looks correct to me. 2708 hold 1024 bytes, so that’s what it should read back.

          The write checksum now includes the 0xFF padding, so will now match the read checksum.

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