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Thread: Appropriate DRAMs for the TRS-80 Model I Expansion Interface

  1. #1

    Question Appropriate DRAMs for the TRS-80 Model I Expansion Interface

    I recently acquired an "as-is" TRS-80 Model I Expansion Interface (final board design) that arrived containing a total of 32K of DRAM, and everything looks really clean an in good shape (capacitors, etc.).

    The DRAM is a mixture of the following chips with a wide variety of date codes:

    • NEC uPD416C
    • NEC D416C-3
    • Motorola 8041016


    The EI is acting a bit flakey (lockups, reboots, various amounts of RAM reported if I get that far), so I wanted to start by making sure that I have the correct type/speed of DRAM installed. I believe that D416C-3 is a good match (150ns), but I'm having trouble finding the data sheets (or any info) on the other two parts. I believe the -3 in the NEC D416C-3 indicates 150ns, and I have seen that -2 is supposed to be 200ns. But on the NEC parts without that last number, I'm not sure how to determine the speed. And I don't see info on the Motorola chip anywhere I have searched.

    Question #1 - Can anyone here confirm whether these are all valid chips to use in the Expansion Interface, whether it's okay to mix/match them in the same EI, and if you know where I could find datasheets for these parts?

    Question #2 - Does anyone know where I could find a relatively low-cost stand-alone DRAM tester for these parts (or a kit or schematic from which I could build one), so that I could test the chips independently from the EI unit? I'm having trouble keeping the system from locking up or rebooting before I can get any meaningful software-based DRAM testing done in the EI itself, so I'd like to be able to test the DRAMs separately. (I see a lot of SIMM DRAM testers, but haven't found one that deals with these chips.)

    Thanks.

  2. #2

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    Quote Originally Posted by kengr View Post
    I recently acquired an "as-is" TRS-80 Model I Expansion Interface (final board design) that arrived containing a total of 32K of DRAM, and everything looks really clean an in good shape (capacitors, etc.).

    The DRAM is a mixture of the following chips with a wide variety of date codes:

    • NEC uPD416C
    • NEC D416C-3
    • Motorola 8041016


    The EI is acting a bit flakey (lockups, reboots, various amounts of RAM reported if I get that far), so I wanted to start by making sure that I have the correct type/speed of DRAM installed. I believe that D416C-3 is a good match (150ns), but I'm having trouble finding the data sheets (or any info) on the other two parts. I believe the -3 in the NEC D416C-3 indicates 150ns, and I have seen that -2 is supposed to be 200ns. But on the NEC parts without that last number, I'm not sure how to determine the speed. And I don't see info on the Motorola chip anywhere I have searched.

    Question #1 - Can anyone here confirm whether these are all valid chips to use in the Expansion Interface, whether it's okay to mix/match them in the same EI, and if you know where I could find datasheets for these parts?

    Question #2 - Does anyone know where I could find a relatively low-cost stand-alone DRAM tester for these parts (or a kit or schematic from which I could build one), so that I could test the chips independently from the EI unit? I'm having trouble keeping the system from locking up or rebooting before I can get any meaningful software-based DRAM testing done in the EI itself, so I'd like to be able to test the DRAMs separately. (I see a lot of SIMM DRAM testers, but haven't found one that deals with these chips.)

    Thanks.
    What you describe sounds like oxidation on the card edge connector of the EI and the Keyboard. I use Deoxit 5 and a QTip to clean them, but finally just got the gold edge connectors and no longer have those issues. Even with brand new identical RAM random lockups, reboots, RAM issues, etc happen.

    I had to clean them once a week because they were in an area with higher than normal humidity.

  3. #3

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    Thanks for your reply.

    I have cleaned the card edge connectors well, and we live in extremely low humidity (southern Arizona), at least until the seasonal monsoons arrive. The connecting cable is brand new and has been tested on a known working system.

    I would like to rule out potentially (a) incorrect DRAM components and/or (b) failing DRAM components, which is why I was focusing (obsessing) specifically about the DRAM part numbers and stand-alone testing.

  4. #4
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    I agree with Tibs and while most systems will operate with a mix-n-match of Ram chips, it can still be hard going if one is flaky. Basically pull all 16 chips and run your memory test program with the EI attached. It will only test the Ram in the keyboard, but the system should be stable. If that succeeds, put one bank of 16K into the EI and repeat. If you have spare Ram chips on hand you can substitute them 1 by 1 until the system settles down, though it can take quite some time.

    Those two NECs and Motorolas are well know as reliable robust chips. Motorolas are what Tandy supplied in most instances (check for a TC logo on each).

    Good luck.

    Ian.
    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  5. #5

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    Thanks for your reply.

    Good to know that these chips are well known as compatible with the EI.

    I don't see a TC logo on any of the chips.

    I will proceed through the elimination process. I do have some spare DRAMs (NEC D416C-3) to swap in, but I don't know if it's good either.

  6. #6

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    8041016 is Tandy's part number for any Mostek MK4116-compatible chip (like the Motorola MCM4116). See page 12 of the 1979 EI service manual.

    Tandy ordered a lot of chips from Motorola that had Tandy's part number printed on them with no Motorola part number. Some of them had a Tandy Corp. TC logo in addition to the Motorola logo, but some just had the Motorola logo and the Tandy part number.

    So, yeah, that chip was produced expressly for use in a TRS-80, so there's definitely not a compatibility problem.
    -Alan

  7. #7

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    Okay, great.

    Thanks for the confirmation on the Motorola part.

  8. Default

    Nice shots of RAM on a Model III board.
    Shows chips with & without the TC stencil.
    Hope this helps.

    20180521_112458.jpg20180521_112450.jpg

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