Re: Drive disconnect switch for SX 64

From: Pete Rittwage <>
Date: Tue, 30 Nov 2010 16:20:59 -0500 (EST)
Message-ID: <>
On Tue, November 30, 2010 4:06 pm, Nate Lawson wrote:
> On 11/30/2010 9:54 AM, Jim Brain wrote:
>> On 11/30/2010 5:17 AM, Marko Mäkelä wrote:
>>> On Tue, Nov 30, 2010 at 11:24:30AM +0100, Niklas Ramsberg wrote:
>>>> Is there a single line that can be switched out to make the drive
>>>> invisible to the computer?
>>> Cutting ATN should do the trick. All communications require ATN to be
>>> pulled down by the controller. If the drive always sees ATN=1, it will
>>> remain passive.
>> Because cbm-hackers is used as a reference resource, I thought I'd add a
>> bit, in case someone sees this later and tries to apply it to a
>> difference situation.
>> Cutting ATN does exactly as Marko notes, the drive is "passive".  But,
>> it is not truly invisible, at least from an electrical perspective.
>> The other lines (DATA/CLK) are still connected to the bus and loading it
>> up (or down, depending on whether the drive is turned on or not).  As
>> such, if you use such an "ATN-killer" switch on a set of drives, the bus
>> could become problematic even though the user thinks many of the devices
>> are "invisible".
>> The cuplrit is the pullup resistors on each drive.  When powered, the
>> resistors are dragging the lines high.  When powered down, they tend to
>> drag the line to ground.
>> For the topic of today (SX-64 drive), it's not a concern, but I thought
>> I'd put it out there in case someone gets the impression they can
>> connect every drive they own and just mark them "off the bus" by
>> switching off ATN.  It won't work beyond a certain number of drives (I
>> think 8 is probably the limit, at least somewhere around there).
> Jim's analysis is good. I've found that with one drive that is off but
> connected to the IEC bus, it doesn't drag the idle level too low (3.3v
> high versus 5v).
> But it's better to always operate an IEC bus with all devices powered on
> and counting each attached device as a load (even if the device isn't
> talking). Don't try more than 4 drives + 1 printer as a max load on a
> single bus, and if using more than 2 devices, try to keep the cable
> length as short as possible.

And some fast loaders will not function with any other devices on the bus
except a single disk drive, correct?

Pete Rittwage
C64 Preservation Project

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Received on 2010-11-30 22:00:28

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