Robert Benschop (rbenschop_at_mac.com) wrote:
> Actually, I always understood that SCSI was a bit risky, serial worse (or
> the other way round, I've been without both for a while, long live Apple ;-)
> but ADB was really tricky, so I always shut down my computers if I needed to
> plug anything in the ADB port.
Robert,
electrically, the SCSI port is probably the most sensitive, followed by
ADB. Serial ports are much more robust and don't mind hot-plugging at
all. Serial communications are very simple, as there can be just one
device at the other end of the line. Peripherals don't have to announce
their presence, and serial drivers can just throw data at the selected
serial port and take it for granted that it will come out right (of
course, individual protocols can add any level of checks to make sure the
device did actually receive the data).
ADB is a bus that accomodates more than one device, so new devices need
to check in. The only clean way of doing this is to connect every ADB
device before powering on the Mac so ADB can detect their presence, but
peripherals attached later will be recognized anyway. We used to have
some fun testing how many mice and keyboards you could attach to a single
Mac -- the old Macs had no problem at all dealing with two keyboards and
two mice simultaneously. (At one time, I had a keyboard, a mouse, a Wacom
graphics tablet, a joystick, and an ADB modem connected to a single
Performa 6320 -- which, much to my surprise, didn't explode.)
SCSI is a bit different again, as SCSI devices need to be assigned a
unique ID. If you don't make a mistake in assigning these IDs, a new
device plugged into a running Mac won't disturb SCSI operations on the
protocol level; on the electrical level, such disturbances can and do
occur, but are rarely fatal. SCSI devices do not announce their presence,
nor does the host adapter poll the bus to detect new peripherals, but
using utilities such as SCSI Probe you can still mount any SCSI drives
added later.
With USB and FireWire, all these considerations should be a thing of the
past, but still I can't just turn on my external CD/RW drive (Formac/
Sanyo) and burn a CD-R -- instead I have to disconnect and re-connect the
FireWire plug before the damn thing is recognized by my Cube. Some timing
problem with the drive's FireWire/Atapi bridge, I guess.
- Michael
Michael J. Hussmann
E-mail: michael_at_michael-hussmann.de
WWW: http://michael-hussmann.de
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