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Thread: Willem programmers and reading late seventies vintage masked ROMS

  1. #1

    Default Willem programmers and reading late seventies vintage masked ROMS

    Hi Guys,

    I'm looking to replace a faulty 2k ROM in the BASIC ROM-PAC for my Sorcerer.
    I had identified the bad chip from examining the 4 ICs in the built-in monitor of the Sorcerer. The faulty chip shows zeros whereas the other three chips have code.

    This is the perfect excuse to unpack and use my Dual Powered Standard Willem EPROM Programmer. From research and talking with other guys from the classic computer list it seems like 2716 or 27C16 EPROMS are suitable replacements for the 2k ROM.

    I've played with the thing for a bit and have successfuly read 27C256 EPROMS and generally familiarised myself with all the bits and pieces. I'd assumed I could read the mask ROM from Sorcerer? I wanted to read them just as a final check to make sure they squared with what I saw with the Sorcerer monitor program. It Willem unit doesn't seem to be able to read them though. All I get is a repeating pattern of 0406 for 256 bytes then 1416 for the next 256 bytes and so on.

    I am sure I have the thing set up right for reading 2716 EPROMS. However I read on the web somewhere that some programmers can't read mask ROMS. Is it likely that is the case here?

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  2. #2
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    Some mask ROMs (I don't know about the ones on the Sorcere) require a CS\ pulse for each byte read; in other words, they don't operate as asynchronous devices. So, a more modern asynchronous EPROM will work to replace them, but not vice-versa.

    What is the part number of the ROMs used?

  3. #3

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    Chunk,

    Here are the numbers on the chip

    AM9218BPC-

    It also says
    31713 78480P (the other chips say 31711, 31712, and 31714 so it refers to it's place in the sequence I guess. The number at the right is the date?)

    and also
    EXSB1-3 (number 3 in the 4 chip sequence)

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  4. #4
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    I don't know anything about the sorcerer either but another anomaly is seen in some some Commodore disk drives, which use ROMs that are 2716-compatible but have a high chip select instead of the normal low CS; in fact the three chip selects (pins 18, 20 and 21) can all be mask-programmed differently than the 2716's low, low, high respectively).

    But in any case it seems odd that you'd get those patterns; I'd expect all FF's or even ascending numbers. I take it that the bad chip wasn't crucial and you could still read it in the Sorcerer itself to see that it was all zeroes? Can the Sorcerer read a 2716?

  5. #5

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    Quote Originally Posted by MikeS View Post
    But in any case it seems odd that you'd get those patterns; I'd expect all FF's or even ascending numbers. I take it that the bad chip wasn't crucial and you could still read it in the Sorcerer itself to see that it was all zeroes? Can the Sorcerer read a 2716?
    Not sure if the Sorcerer can read a 2716. I guess I'll soon see (-:

    The offending chip was in the ROM-PAC. During the boot up sequence the computer checks the number 4 chip. If it sees nothing it assumes there is no ROM-PAC inserted and defaults to the monitor program. I removed the number 4 chip (essentially disabling the auto start) then looked at the contents of the remaining chips with the monitor. Chips 1 and 2 had code and chip three had all zeros!

    I then removed chip 3 and put chip 4 in the chip 3 socket. I did this to check the ROM was ok, (and ascertain the number 3 socket was also ok). Looking at the contents showed code, so my conclusion was it was chip number 3 that was causing the ROM-PAC to hang the machine.

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  6. #6
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    Tez, the part number helps a lot. This is a JEDEC-style mask-programmed ROM, but there are three, count 'em, three chip selects (see, for example, its pin-compatible Motorola equivalent, the MCM68A316E) The selects are on pins 18, 20 and 21.

    Now for the tough part--they are mask-programmable, just as the data cells are. So a chip-select can be any one of 8 possibilities. It's a pretty slow chip--150 nsec delay between CS asserted and output.

    Hope this helps.

  7. #7
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    Quote Originally Posted by Chuck(G) View Post
    ...a chip-select can be any one of 8 possibilities.
    Hrmph! That's what I just said

    But I still think if it's a CS issue he'd just see FF's, unless there's something unusual about that Willem...

  8. #8
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    Okay, Mike--you were first. I was more intent on finding a datasheet for the 9218 and missed your observation.

    It could be that this is an edge-selected read cycle. So you'd pluse CS every read cycle. I've certainly seen those ROMs.

  9. #9

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    Thanks guys,

    I'm sure it's some problem like that. The Willem can certainly read 27256 ROMS ok and it SAYS it can handle 2716s. It is one of these cheap generic ones though.

    I guess for me the important thing is that I should be able to program a 27C16 with the binary I've got and it should work in the ROM-Pac.

    This is all new territory for me. Actually it's less daunting than I thought it would be.

    Mike at some stage I want to use the burner to replace a couple of my PET ROMS, to get my second board up and running. Can the standard Willem programmer read these ROMS?

    Tez
    ------------------------------------------------
    My vintage collection: https://classic-computers.org.nz/collection/
    My vintage activities blog: https://www.classic-computers.org.nz/blog/
    Twitter: @classiccomputNZ ; YouTube Videos: (click here)


  10. #10
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    Quote Originally Posted by tezza View Post
    I guess for me the important thing is that I should be able to program a 27C16 with the binary I've got and it should work in the ROM-Pac.
    Tez,
    From the barely legible schematic of the ROM PACK, it seems to me that all three enables (pin 18,20 & 21) are active low. So it looks like the 2716 will not work as-is.

    You may need an adapter that connects pin 21 of the 2716 to Vcc and not to pin 21 of the ROM socket which is grounded along with pin 20.

    Here is the tech manual. The ROM pack is the first schematic. See if you read it the same way. Note that the parts list on P.46 lists the parts as 2716 "PROMs" which is very confusing.

    -Dave

    Sorcerer Technical Manual
    Last edited by dave_m; June 18th, 2011 at 09:20 PM. Reason: added parts list info

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