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Thread: Mcm6574 character generator ic

  1. #1

    Default Mcm6574 character generator ic

    This IC, used in the SOL-20 and Processor Technology's separate S-100 video card is fairly rare now, so I thought I would post a link, they are stocked by Kevin Keinert and listed on his rare IC pages, just in case anybody needs one:

    http://www.gameroomrepair.com/ICparts/ICparts.htm

  2. #2

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    I have also had to replace this part recently (in a Processor Technology VDM-1 board). I found the MCM66740 is a drop in replacement for the MCM6574 - even though it doesn’t seem it would be at first glance. Presently a seller has a number of them for sale on eBay. I bid $6.00 each for three pieces and he accepted.

    Mike

  3. #3
    Join Date
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    Quote Originally Posted by deramp5113 View Post
    I have also had to replace this part recently (in a Processor Technology VDM-1 board). I found the MCM66740 is a drop in replacement for the MCM6574 - even though it doesn’t seem it would be at first glance. Presently a seller has a number of them for sale on eBay. I bid $6.00 each for three pieces and he accepted.

    Mike
    Just ordered some myself. Figure I have enough Sol-20 and VDM-1 boards to justify having a few around.

    Cheers,
    Corey

  4. #4

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    Thanks for the heads-up on the MCM66740 (and the MCM6574)!

  5. #5

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    Hi,

    Great to find a ROM with an identical character set.

    I'm a little baffled on how it could be a drop in replacement, for example on the 66740 pin 1 is /CS3, pin 3 /CS4, pin 10 /CS1 and pin 14 /CS2. With the 6574 they are VBB,VDD,N/C & N/C respectively. I don't have the Sol mobo out of the case currently to check the connections that are N/C, but I checked a VDM-1 board, and pin 10 for example is N/C, which would leave that /CS1 pin floating on the 66740, when it should be tied high. So ideally that should be done on the VDM/1 board and possibly, the Sol or an adapter made. Pin 14 on the VDM-1 also looks N/C on the pcb. Also I have not checked the implications of the other /CS pins vs the voltages applied to VBB & VDD. So as you say the 66740 may work just plugged in for the 6574, but there could still be issues..worth checking.
    Last edited by Hugo Holden; January 4th, 2019 at 04:40 PM.

  6. #6

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    Quote Originally Posted by Hugo Holden View Post
    I'm a little baffled on how it could be a drop in replacement, for example on the 66740 pin 1 is /CS3, pin 3 /CS4, pin 10 /CS1 and pin 14 /CS2. With the 6574 they are VBB,VDD,N/C & N/C respectively. I don't have the Sol mobo out of the case currently to check the connections that are N/C, but I checked a VDM-1 board, and pin 10 for example is N/C, which would leave that /CS1 pin floating on the 66740, when it should be tied high. So ideally that should be done on the VDM/1 board and possibly, the Sol or an adapter made. Pin 14 on the VDM-1 also looks N/C on the pcb. Also I have not checked the implications of the other /CS pins vs the voltages applied to VBB & VDD. So as you say the 66740 may work just plugged in for the 6574, but there could still be issues..worth checking.
    Yes, these are the exact concerns I had that led to my comment above: that at first it didn't seem it could work as a replacement. However, when I finally found the full datasheet in a Motorola databook (I have extracted both datasheets and posted at http://deramp.com/downloads/processo...l-20/hardware/), it became clear that the 66740, and several other 667xx parts, are direct replacements for several of the 657x parts.

    These parts are mask ROMs and the chip selects be set to 0, 1, or "no-connect." On page 2-121 of the 667xx data sheet it states that in all the standard part numbers of the 667xx series (other than the 66751), all four chip select pins are set to no-connect. I verified the no-connect when I received the part. I can find no connection - including any intrinsic diode - between the chip select pins and any other pins on the part.

    On page 2-130 of the 667xx data sheet is a table that maps 657x parts to 667xx parts.

    Mike

  7. #7

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    Quote Originally Posted by deramp5113 View Post
    On page 2-121 of the 667xx data sheet it states that in all the standard part numbers of the 667xx series (other than the 66751), all four chip select pins are set to no-connect. I verified the no-connect when I received the part. I can find no connection - including any intrinsic diode - between the chip select pins and any other pins on the part.

    Mike
    Mike, That is great, Thanks!

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