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8" Floppy Drive Termination Question

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    #16
    Originally posted by mkstabd View Post
    Thanks to all for the responses.

    @MikeS: I hadn't considered terminating any other drive than the last drive in the chain. I'll have to try that out.
    Should work.
    Also, I updated the diagram to switch on the +5V side vs the ground side for the "enabled" mode. Please help me understand why sharing the drive ground is a bad thing. Do I need to look at a completely isolated +5V source? I was hoping to use the +5V source already in the enclosure. I thought I would be using the same ground that the chip resistor terminators normally plugged into the drive were using. I'll take another look at the drive schematics.
    Sharing the ground (and +5V) is a good thing; I'm just pointing out that switching the ground as you're still doing is a bad thing, although in this case the switch probably won't do anything anyway except possibly introduce noise since the grounds are still connected together through the power supply.

    Also, the way you have the buffers wired defeats their purpose; you'd connect all the inputs to +5V and connect the switch to the enable inputs (also connected together); I don't think key bounce would be a problem

    But I still suspect that none of this is really necessary.

    BTW, what are you using to draw those diagrams? I'm always looking out for new tools.

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      #17
      Thanks for the response, MikeS

      Originally posted by MikeS View Post
      Sharing the ground (and +5V) is a good thing; I'm just pointing out that switching the ground as you're still doing is a bad thing, although in this case the switch probably won't do anything anyway except possibly introduce noise since the grounds are still connected together through the power supply.
      You've made me realize something: I was only thinking of operating the switch with everything powered off; it hadn't occurred to me to flip the switch when the bus was powered up. If the switch was flipped with power, what would the worst case scenario be? Could the noise you describe fry any chips in my floppy drives or host controller? Or would the effects be more like corrupting data transfers if any were in progress at the time?

      Also, the way you have the buffers wired defeats their purpose; you'd connect all the inputs to +5V and connect the switch to the enable inputs (also connected together); I don't think key bounce would be a problem
      LOL @ key bounce

      The tri-state buffer schematic was my attempt at capturing what I thought Chuck was describing, which was to isolate the resistors from each other when that side of the circuit was powered down (switch in disable/pass thru mode.) If there was no +5V flowing to the buffers, then all the paths between the resistors would be blocked. Conversely, if there was +5V to the buffers, then they'd activate, allowing the resistors to rejoin the circuit.

      It's kind of like an anti-fix in that their primary purpose was meant to support no signal flow, rather than allowing signal flow. In this case, the buffers achieve the same effects as using the schottky diodes, but with no voltage drop. Perhaps not the most efficient use of the buffers, but I still think it's a way cool idea!

      BTW, what are you using to draw those diagrams? I'm always looking out for new tools.
      I threw those together with Visio. Can you recommend a good freeware modeling tool?

      Ty

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        #18
        Originally posted by mkstabd View Post

        LOL @ key bounce


        Yeah, that was dumb. I misread the previous post.

        Duh!

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