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Thread: Memory Compatibility Z80 vs 8080

  1. #11

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    Quote Originally Posted by Hugo Holden View Post
    Maybe the Yang board has a fault or needs the refresh signal of the Z80 system, I will investigate.
    If the Yang board uses the 2Kx8 6116 RAMs then it is a static RAM board which does not use any sort of refresh.

    Mike

  2. #12

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    Quote Originally Posted by deramp5113 View Post
    If the Yang board uses the 2Kx8 6116 RAMs then it is a static RAM board which does not use any sort of refresh.

    Mike
    Thanks Mike. Of course.

    So now there are two ways to spot a likely compatible RAM board for the SOL - 20:

    If its DRAM it must not use pin 66.
    If its SRAM its ok.

    I'll report what was wrong if I can repair the Yang board.

  3. #13

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    I recall making a small board with a TTL gate that I added to my Poly88 CPU card to handle a SD DRAM board. It was an 8080 processor. It just needed to tell the SD card when to do refresh. Some boards expect there to be an external refresh address counter, like the one you were looking at. This was a build in feature of the Z80 as it would automatically generate the address for 128 cycle refresh( but not 256 cycle as some 64K DRAMs required a full DRAM controller ).
    Your static board should work fine with a 8080 processor.
    At the VCFNW, I bought a 64K static board for $5 . I removed the SD card I had in my Poly88 and used it at the LAST Maker Faire
    Dwight

  4. #14
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    Dwight, there are a couple of ways to extend the 7 bit refresh counter to 8 bits on a z80 without going to a full DRAM controller.

  5. #15

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    Do note that some banked cards need a write to some port address to be enabled. This should be easy to defeat once identified.
    The S100 buss was designed to have one set of 8 bit lines for writing from the processor as data and another set of 8 bit lines for data returning to the processor. Some RAM cards were intended to be used with a 16 bit system and used both 8 bit busses for a bidirectional 16 bit bus. Look at how the bus buffers are connected.
    Dwight

  6. #16

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    I have repaired the Yang board. And it works in the SOL-20. There were two issues. It has jumpers for normal S-100 mode addressing and another mode used with the YES-5 computer. But the silk screen labels on the board for the settings are incorrect for four of the jumpers, so it was not responding to the correct addresses. I figured this out from the schematic of the address decoder where it is correct and the manual is not wonderfully helpful otherwise. Once it started working then I discovered that one of the 16 HM6116P IC's was defective.

    Are the HM6116P's thought to be a reliable IC or do they often require replacing or is it rare for a failure of these types?

  7. #17

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    There is a little oddity on this Yang board. There is an indicator LED which it driven on when any of the 16 memory chip select lines are active, the odd thing is that the designer didn't put a current limiting resistor in series with the LED and just relied on the gate's ability to sink current being limited, not a wonderful idea I would have thought. Has anyone seen LED's driven like this from TTL circuits before, with no resistor ? Circuit fragment attached, there is no resistor on the actual board either.

    Also, looking at the jumpers on pin 5 of U9 it is possible if two were there, to short out the +5V rail. This probably explains why there were separate soldered jumpers , even though the silk screening on the board said DIP switch.It probably wouldn't have been a bad idea to have put a small series resistor on the connection to +5v.
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    Last edited by Hugo Holden; August 9th, 2019 at 05:44 PM.

  8. #18
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    That is strange--you're depending on the resistance of the pulldown transistor in the output totem-pole datasheet. Maybe it's enough to avoid toasting the LED. Oddly, you'd be better off reversing the LED and connecting the other side to ground, as the high-side output transistor has a 120 ohm resistor in series. Can't say that it would work well with a 74HC04, however.

  9. #19

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    Quote Originally Posted by Chuck(G) View Post
    That is strange--you're depending on the resistance of the pulldown transistor in the output totem-pole datasheet. Maybe it's enough to avoid toasting the LED. Oddly, you'd be better off reversing the LED and connecting the other side to ground, as the high-side output transistor has a 120 ohm resistor in series. Can't say that it would work well with a 74HC04, however.
    Double checked the device actually on the pcb its an LS04. I probably should put a 100R resistor in there at least, but I would have to modify the board, cut a track etc... hmmm.... maybe just leave it as the curio that it is.

  10. #20

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    If you hack in and measure current through the LED, you’ll find it’s less than you think - probably in the 20-40ma range.

    Mike

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