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Thread: TRS 80 Non Gate array

  1. #1
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    Default TRS 80 Non Gate array

    I was just curious. In order to speed up a Model 4 Non Gate Array would you bend pin 7 on U3, bend pin 27 on the Z80 CPU, replace the non gate array motherboard with a gate array motherboard, or get one of the rare speed up boards?
    Also another thing, could it be possible that some Model 4 gate array still had the downward facing rs232 port?

  2. #2

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    Model 4 NGA you can simply ignore U3, Pin 7 and just bend out Z80 Pin 27 , that will take out the wait state out. Price... 0$, but it does NOT accelerate your Model 4, it just gives you pure 4MHZ.
    Replace NGA with GA board... ebay 150$ still at best 4-5MHZ
    Xlr8r Board - good luck finding one, likely found inside a computer you purchase, no "3rd party " making them right now , Ian is looking into getting Pascal to make new pcb from a complete functional board - Price TBDL
    and if you could find one you can get from 6-8 MHZ under certain specification of Ram speeds.

    my vote , choice #1

    on your last question , no all GA board have the angled rs232c port on them. and NGA board have the edge card, at least that I am aware. easy to figure out, look at the board and what is the revision of the board A,B, C, D is NGA.
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  3. #3
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    Why do the NGA Model 4's have such a wait state and why would bending a pin remove it?

  4. #4
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    When the Z-80 reads an instruction opcode it uses 2 cycles (or T-states). But when it reads data it uses 3 T-states.

    Early Model 4's came with DRAM that was only fast enough for 3 T-state access. So they added a circuit which detects the 2 T-state opcode fetch (based on the /M1 signal) and triggers a single wait state so the access would take 3 T-states and hence would function properly.

    The bending out of the pin disables this /M1 detection and wait state generation. I don't know how exactly it does that.

  5. #5
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    So, I have a Model 4 Gate Array and I've read the 'speed up' stuff in the past, but I'm curious why one would do that now? I've never bothered because I haven't noticed anything be particularly slow, so wondering if folks are doing that 'just to do it' or if there is some specific noticeable slowness that they're experiencing that is initiating wanting to do this? Also, if one were to make this modification, is there some software that this would negatively affect? (for instance, games become somewhat unplayable?).
    -- Brian

    Systems: Amstad PCW 8256, Apple IIe/II+/GS/Mac+/Mac 512k, Atari 800/520STFM, Commodore 64/128/Amiga 3000/PET 4032/SX-64, IBM PS/1 2121-B82, Kaypro II, Osborne 1, Tandy 1000 SX, TI-99/4A, Timex Sinclair 1000, TRS-80 Color Computer 3/Model 4 GA

  6. #6
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    So would this disable Model 1 compatibility?

  7. #7
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    It won't affect compatibility. The wait states are only applied when the Z-80 is set to 4.055 MHz (i.e., "Model 4 mode"). If the CPU is at 2.02752 MHz ("Model III mode") there are no wait states so the modification will have no effect.

  8. #8
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    Out of curiosity would replacing the Z80 with lets say a Hitachi HD64180 or Z180 or Z80182 or Z80B increase the clocks of the non gate array from 3.5mhz to the full 4mhz.
    Last edited by abruno17; March 30th, 2020 at 01:52 PM.

  9. #9
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    Quote Originally Posted by gp2000 View Post
    The wait states are only applied when the Z-80 is set to 4.055 MHz (i.e., "Model 4 mode").
    What about model 4 software compatibility?

  10. #10
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    Quote Originally Posted by abruno17 View Post
    Out of curiosity would replacing the Z80 with lets say a Hitachi HD64180 increase the clocks of the non gate array from 3.5mhz to the full 4mhz.
    No. It will still obey the wait-state input and be slowed down.

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