Re: Grounding on the //gs

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

Lockar wrote:

>      I noticed today that one of my //gs's might have a grounding
>issuse.  I'm not sure, maybe you guys let me know what I should be
>looking for.
>
>      I went to plug the a 3.5 drive into the unit (with it off mind
>you, but still plugged in) and I taped the sides of both contectors (I
>missed plugging it in, and the two sides hit each other) and there was
>a small spark.  At first I thought it was stactic (which isn't good, I
>know.)  But it did it again, so I unplugged it and looks around, but
>every thing 'seems' find but I don't know.
>
>     Is there maybe a grounding wire I'm not seeing. I have replaced
>//gs mother boards before and I don't remeber there being any grounding
>wire.
>
>   What do you guys think?  Or is this //gs toast? :(

It must be static, unless you have connected the 3.5" drive to
something.

Normally, a 3.5" drive is connected only to an Apple and to
other drives (daisy chained), so there should be no possibility
of a ground loop of any sort (which would require that something
external to the Apple, in the daisy chain, was somehow connected
to the AC power--but no Apple drive is separately powered).

The bad news is that unless the shield of the connectors made
the spark, the discharge may indeed have toasted either the
interface IC in the drive or the IC in the IIgs, or both.

The good news is that the connectors are constructed so that
the shields make contact before any of the signal lines.  But you
mention putting tape on the "sides"--does this mean that you
insulated the metal shields?  This is a bad idea!

Except in the power supply, "grounding" is a relative term.
What is required is that the "ground reference" of all components
be the same--that is, metallically interconnected.  Since there
can easily be a significant difference in potential of unconnected
components, it is a good idea to bring the components to nearly
the same potential before completing the metallic connection,
usually by touching "shield" metal parts of both components
simultaneously before making the connection (so that your body
makes a resistive connection between parts of the components that
can withstand a static discharge).

-michael

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