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Thread: Booting from an external floppy on a model III

  1. #11
    Join Date
    Jun 2013
    Location
    Melbourne, Australia
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    159

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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
    Join Date
    Jun 2013
    Location
    Melbourne, Australia
    Posts
    159

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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
    Join Date
    Jun 2013
    Location
    Melbourne, Australia
    Posts
    159

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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.

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