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From: jbr@key.COM (Jeff Rubin)
Newsgroups: comp.sys.mac.hardware
Subject: Re: ADB and blown motherboards
Summary: Summary of replies
Message-ID: <1511@key.COM>
Date: 6 Mar 90 19:46:49 GMT
References: <1494@key.COM> <1738@sequent.cs.qmw.ac.uk> <2848@mtuni.ATT.COM>
Reply-To: jbr@hongkong.key.COM (Jeff Rubin)
Organization: Key Computer Laboratories, Fremont
Lines: 64


Thanks to those who emailed responses to me about playing with ADB
devices with power on.  Here is a summary of the responses:

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From: dwade@jarthur.Claremont.edu

	ADB devices are indeed powered on.  I've never had a hardware problem
as a result of unplugging ADB devices, but I have gotten crashes rarely and
generally the Mac just refuses to recognize the device.  I am under the
impression that a fair amount of information is exchanged at startup between
the keyboard and the Mac, so presumably this information is lost when the
device is unplugged.
	I know that if you plug two Macs into one keyboard you're asking for
lots of problems, including hardware problems...
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From: rrkasegu@ATHENA.MIT.EDU

[Does the triangle power on key use the ADB?]
No, it doesn't.  Pin 2 of the 4-pin ADB connector is connected to the 
ground pin of the ADB connector when the triangle button is pressed.  The
power-on circuitry detects when that pin is grounded (the power on button
does the same, I believe), and turns on the power.  The power line is not
live when the power is off, nor is the ADB circuitry on the main board.
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From: dplatt@coherent.com

The main ADB-bus power line is run through a small surface-mount fuse
soldered to the motherboard (on the Mac II, at least).  If you plug or
unplug an ADB connection with the power on, there's some chance that
you'll accidentally short the plug's ADB-power pin to the metal shell on
the jack... which is grounded.  This will dump +5 through the short, and
blow the fuse on the motherboard.  If your Mac is not under warranty or
AppleCare, you can simply tack-solder a small fuse-holder to the ends of
the fried chip-fuse, and install a user-replaceable fuse... I've heard
from people who have done this.

The "power on" key is not tied into the main ADB power, nor does it use
the standard ADB data-transfer mechanisms..  It's a simple electronic
mechanism, driven by the Mac's battery through a _very_ large resistor.
When you press the power-on key, it deliberately grounds the power-on
line on the ADB; a sensor in the Mac detects the fact that a very small
amount of current is flowing, and triggers the power-supply-activation
logic.  I don't believe that accidentally grounding this lead will do
any harm to the Mac circuitry... it'll just trigger the power-on
process.  The power-on circuit draws _very_ little current... you can
actually force the Mac to boot by simply bridging the switch contacts
with a slightly sweaty fingertip!

If you were to accidentally short _both_ the ADB power line and the
power-on line to ground simultaneously, you might possibly do some
damage, since the Mac would power up and then blow the ADB fuse... but
I'm not certain that this is physically possible given the design of the
mini-DIN plug.  You might want to pull the AC plug, just to make triply
sure... but this is probably carrying caution to the point of excess.
I've NEVER heard a report of anybody frying their ADB if they had
powered the Mac down before fiddling cables.
------------------------------------------------------

Again, thanks to all.  It would seem this is a case of RTFW.
(The W is for warranty)
-------------------------------------------------------------------
Jeff Rubin			"I thank you from the heart of my bottom" -anon
...!amdahl!key!jbr