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Thread: How to burn 2708s

  1. #101
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    Quote Originally Posted by daver2 View Post
    The programming software should disconnect all the power supply voltages from the EPROM when it has finished reading from or writing to it.

    Likewise, when the computer is turned on, the programmer design should be such that voltages are not applied to the EPROM socket.

    This should permit you to insert and remove devices from the ZIF socket without powering down your computer every time!

    When I built a 2708 EPROM programmer many years ago, I included red LEDs to indicate the varying voltages. The rule was to only remove or insert an EPROM when all the LEDs were OFF.

    Dave
    My board has a High Voltage switch with an led that turns on when you switch it. The software prompts you to set it on and off for programming. I had assumed the ZIF socket was always live because it seemed to be reading the eprom even with the HV switch off. But maybe its reading nothing. I coud swear I had the switch off and it read the contents of the nearly perfect EPROM back to me once though..

  2. #102

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    Quote Originally Posted by falter View Post
    My board has a High Voltage switch with an led that turns on when you switch it. The software prompts you to set it on and off for programming. I had assumed the ZIF socket was always live because it seemed to be reading the eprom even with the HV switch off. But maybe its reading nothing. I coud swear I had the switch off and it read the contents of the nearly perfect EPROM back to me once though..
    The switch only works with the programmer initialized and powered up. When you power on or off your computer, It can be in almost any state to the 2708. Look at the schematic. The switch doesn't block the programming voltage, it controls a 7406.
    Not all programmers turn off the VCC, when not reading the chip or programming. Most do but it is not essential. This programmer doesn't turn off either power level. It should hold the programming voltage off though. You should check this at the socket. With the switch off there should be no high voltage at the programming pin when reading. With the high voltage switch on and reading, there still should be no high voltage on the programming pin. The only time there should be a voltage is when it is actually programming and even then, it should be pulsing to about 25V.
    Since the 7406 doesn't control the voltage unless it is powered up, it is quite likely you are programming the part when you turn your computer on and off.
    Anyway, try erasing and first turn on your computer. Start your program to initialize the 6821. Then put the EPROM in the socket. and program it. If you still see FF going to 00, it means that it isn't controlling the programming voltage. This could be the 7406, transistor, the diode or even the resistor that connects the high voltage to the base of the transistor.
    The fact that it is programming the last location indicates that the programming voltage is on when it should not be. This can happen, independently of the switch position, it there is a failure of the parts I mentioned or at turn on/turn off of the computer.
    Dwight
    Last edited by Dwight Elvey; January 23rd, 2018 at 03:04 PM.

  3. #103
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    Okay. I think I will clean the ZIF socket though first (if that's possible). I played around some more and found if I moved the chip around in the ZIF and then looked at memory contents, it got closer to what I was looking for.. ie. it had the first few addresses correct and then went off, then correct again and then off some more. It changes based on how I've got the EPROM positioned in the socket (either more towards left side or right).

  4. #104
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    Okay this time it worked! Brushed of the ZIF socket 'fins' a bit. Erased the EPROM for 10 min, fired up the computer. Loaded the CHIPOS data into RAM from my S file. Plugged the chip into the ZIF socket, fired up the burning software and burned (with turning the high voltage on and off as prompted). No problems at all. Then I exited the software, removed the EPROM and powered the machine off. Powered it back on, inserted the EPROM, fired up the software, transferred contents to memory and verified. It's correct all the way through.

    Whether it'll do this again is anyone's guess -- I should probably try it with that other EPROM I have just to see if that EPROM was really dead or not. But yay, I have my CHIPOS for this Dream 6800 I plan to build once the TVT is done. Can't wait.

  5. #105
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    If you don't already have it in your toolkit, invest in an can of Caig "DeoxiT" DN5 (aerosol spray) or CRC "QD Contact Cleaner" (your choice). Either is really good and useful for oxidized metal contacts.

  6. #106

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    Quote Originally Posted by falter View Post
    Okay this time it worked! Brushed of the ZIF socket 'fins' a bit. Erased the EPROM for 10 min, fired up the computer. Loaded the CHIPOS data into RAM from my S file. Plugged the chip into the ZIF socket, fired up the burning software and burned (with turning the high voltage on and off as prompted). No problems at all. Then I exited the software, removed the EPROM and powered the machine off. Powered it back on, inserted the EPROM, fired up the software, transferred contents to memory and verified. It's correct all the way through.

    Whether it'll do this again is anyone's guess -- I should probably try it with that other EPROM I have just to see if that EPROM was really dead or not. But yay, I have my CHIPOS for this Dream 6800 I plan to build once the TVT is done. Can't wait.
    You missed a possibly important step. You didn't say you initialized the programming software. It puts the 6821 in a proper state.
    Dwight

  7. #107
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    Oh I wrote that wrong. I fired up the software *before* putting in the chip. Then I did the burn command and switched on the high voltage when requested.

    I'm tempted to try the other 'bad' 2708 I had.. see if I get the same result, or if my method was the problem the whole time.

  8. #108

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    The programming of the FF address was likely the powering down with the chip in place. As the 5V was going down, I suspect the 7406 lost control of the transistor that turned on the high voltage. There was likely enough charge on the capacitors of the charge pump to program the last address.
    I don't know the states that the 6821 come up in during reset. That may have been a issue then as well.
    The flaky socket wasn't helping any either.
    Dwight

  9. #109

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    I was looking at the schematic:

    http://www.swtpc.com/mholley/MicroWo...hematic100.jpg

    If they'd moved the switch to ground to U5-2 and swapped the on off label on the PC board, it wouldn't bother it to turn the PC power on and off while the 2708 was plugged in.
    Dwight

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