Re: Drive disconnect switch for SX 64

From: Jim Brain <>
Date: Tue, 30 Nov 2010 11:54:38 -0600
Message-ID: <>
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.
>     Marko
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 Brain, Brain Innovations                                      (X)
Dabbling in WWW, Embedded Systems, Old CBM computers, and Good Times!

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Received on 2010-11-30 18:00:32

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