Re: ADB Switchbox requirements

[mjmahon@aol.com (Michael J. Mahon)]

Bill Garber wrote:

>"ToasterKing" <toasterking@SPLATbigfoot.com> wrote in message
>111020031335239425%toasterking@SPLATbigfoot.com">news:111020031335239425%toasterking@SPLATbigfoot.com...
>> In article <20031010064354.15639.00000094@mb-m13.aol.com>, Michael J.
>> Mahon <mjmahon@aol.com> wrote:

<snip>

>> > >  A standard ADB cable
>> > >should work fine.  Hint: If you don't have spares, you can also use an
>> > >S-Video cable in place of an ADB cable -- they're identical as far as
>> > >connectors and shielding.
>> >
>> > Well, not quite identical--when you use an S-Video cable for ADB,
>> > you put the ADB signalling on one of the internal shields, and +5v
>> > on a center conductor.  It works, since ADB is low speed, but
>> > it isn't made like an ADB cable.
>>
>> I stand corrected -- so shielding isn't the same.  If that's the case,
>> I can see how the shielding of an S-Video cable wouldn't be as
>> effective, since the shield is not carrying signal ground.  That
>> doesn't seem to be too much of an issue, though.  Like you said, ADB is
>> low-speed and as such isn't as prone to noise or even very short logic
>> glitches.  Apple also says the maximum cable length of the ADB bus
>> should be 5 meters.  In the past, I've used cabling about 6.1 meters in
>> length involving 2 devices plus a switchbox (that's cabling to one
>> computer), and 3.7 meters of that was S-Video cabling -- and it still
>> always worked fine.  But thanks for the heads-up.
>
>You could always pass a small resistor between signal ground and
>shield ground in the switch box rewarding you with that extra amount
>of protection.

There is no distinction in an S-Video cable.  It is just two coaxial
cables with one shield connected to the ADB signal pin.  The
shield on that coax _is_ the signalling conductor, and the inner
conductor is at AC ground (+5v).  This unconventional arrangement
is functional at low speeds, but it remains unconventional.

Adding resistors would only load the bus, possibly upsetting the
built-in termination.  It might work, but it won't improve anything.

-michael

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