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Thread: Booting from an external floppy on a model III

  1. #11
    Join Date
    Jun 2013
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    Melbourne, Australia
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    161

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    Quote Originally Posted by mrthreeplates View Post
    I'll report back my progress with pictures.
    Nice! I'm looking forward to seeing your solution.

  2. #12

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    Success!

    I used a small prototype board from chipquick as space was super tight. As I was worried about adding additional power to the system, I used low power cmos ttl compatible chips (HCT). Soldering was kind of tricky.

    With the switch disabled, my drive assignment is unaffected as expected.

    I flipped the switch, rebooted, and had the external drives swapped with the internal ones. Doing this allows me to boot directly off of my hxc emulator while still accessing the internal floppies.

    Since the only mod to the floppy controller is adding a socket for u6, this mod is reversible.

    My next step is to build a pcb. I have another trs80 and I'd like a second one of these boards. I think it would be cool to make the mod even smaller and I want an excuse to make a pcb anyway.

    Comments or suggestions for improvements appreciated!

    IMG_20190810_134855.jpgIMG_20190810_131457.jpgIMG_20190810_174514.jpgIMG_20190810_174411.jpgIMG_20190810_173130.jpgIMG_20190810_152355.jpgIMG_20190810_151725.jpgIMG_20190810_143521.jpgIMG_20190810_145849.jpgIMG_20190810_134147.jpgIMG_20190810_134241.jpgIMG_20190810_131824.jpgIMG_20190810_223221.jpg

  3. #13
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    Jun 2013
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    Melbourne, Australia
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    Nice! So what are the advantages of using the 74157 over just using a 4PDT switch?

  4. #14

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    I am not sure there are any advantages as they are functionally the same (maybe less noise?). I just thought it a bit more elegant to use a simple logic switch vs. extending the drive select lines through a big physical switch. And I wanted to see if I could do it... just a software guy who is dangerous with a soldering iron!

  5. #15
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    Jun 2013
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    Melbourne, Australia
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    I suspect the 74157 approach would be better if you needed to switch the drives while the computer is on (assuming that's a good idea in the first place). I suspect the physical switch can't be guaranteed to cleanly switch all the lines at the same time.

  6. #16

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    A Thanksgiving update

    In the spirit of "what is with doing is worth over-doing" , I went ahead and tried to create a pcb for my working prototype. Besides, I need another board for my model 4 ga since it works for my model 3.

    After spending an afternoon learning KiCad, I think I have a solution. I paid my $10 and oshpark should send me 3 boards!

    You have to understand that the last time I tried to create a pcb it was with Radio Shack's etching kit back in the 1970s. Given I saved up many weeks of earnings mowing lawns, you can imagine my disappointment when my fantasies of creating my electronics projects turned out so badly. All of my traces rotted and the results were useless. This was an expensive lesson for me. It didn't help that I had the dexterity and patience of a 10 year old. It all ended up the the trash.

    So, fast forward to today when I can use amazing software to create my projects for nearly free. Very cool!

    Is this necessary? Definitely not.

    Is this a waste of time? Totally.

    Is this fun? Absolutely!

    The hard part is waiting for oshpark to ship me the results. I feel 10 again... some things don't change...

    Note: I really would like to try another version using soic parts for an even cleaner solution... we'll see how this turns out and maybe I'll do another version...
    Attached Images Attached Images

  7. #17
    Join Date
    Jun 2013
    Location
    Melbourne, Australia
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    Thanks for the update. It's looking good. It's never a waste of time if you're learning things and having fun.

    I was thinking an interesting modification would be to come up with a system where the external switch isn't required, e.g. something like:
    (1) When power is turned on, it defaults to the internal drives being 0 and 1.
    (2) When the reset button is pressed with a certain key on the keyboard pressed down (or maybe just if the reset switch is held down for a certain amount of time), it triggers the circuit to toggle to the external drives being 0 and 1.
    (3) It remembers the setting in between normal resets.
    (4) The trigger can be used to toggle between states.
    (5) When the power is turned off, it forgets the setting to avoid needing some form of memory, and to avoid people thinking the computer is broken if they don't know the circuit is installed.

    To achieve this it will need to monitor the RESET line and possibly keyboard matrix lines depending on which way the trigger is done.
    Last edited by gonk23; November 29th, 2019 at 11:23 PM.

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