Termination needs to be applied to both physical ends of a SCSI bus,
regardless of whether they are physically inside or outside the machine, or
whether one of them happens to be the controller. When both internal and
external cables are present, generally the outer ends of each cable
represent
Can I see the picture in question ?
Sent from my iPhone
On May 7, 2014, at 7:40 AM, Wesley Furr wes...@megley.com wrote:
Quick question...I recently saw a photo of a Mac LC motherboard (the original
one) that had a small board attached to the internal SCSI port...from what I
could see
if that
is not a correct assumption.
_
From: vintage-macs@googlegroups.com [mailto:vintage-macs@googlegroups.com]
On Behalf Of Jonathan Morton
Sent: Wednesday, May 07, 2014 7:48 AM
To: vintage-macs@googlegroups.com
Subject: Re: MAC LC and Internal SCSI Termination
Termination needs to be applied
Most 68k Macs have the very simple NCR5380 SCSI controller, which most
assuredly does not have automatic termination. The LC certainly doesn't
have anything better than that.
- Jonathan Morton
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OK...so let's assume we have an LC with an internal drive and nothing
else. The drive and the controller are terminated such that both ends of
the bus have proper termination. When you plug in an external SCSI device
such as a ZIP drive, what do you change to disable termination on the
You don't change anything. With a single internal drive fitted, the bus is
physically short enough to require only one terminator, which matches the
default configuration of a Mac.
The fundamental reason for termination is that electric signals tend to
bounce off the end of a transmission line,
Interesting...I just learned something new. I've always had it drilled
in, termination, termination, termination... Also used to the PC world
where there is either automatic termination, or terminating resistors that
can be installed or removed.
So...the LC models that shipped without an
It may be worth remembering that for most of the original lifetime of 68k
Macs, PCs did not even have PCI buses. This was an earlier era of computing
than you may be thinking of, in terms of automatic termination. Selectable
and pluggable termination was, however, commonly in use on both
Indeed...PCI was getting going good in the mid 90's (Wikipedia says it was
created in June of 1992), and the LC dates to around 90-91 I think? I was
going with the assumption that both ends must be terminated, and if
there's no selection for on-board termination, then it must be automatic.
I
I can confirm that diskless classic Macs did not (except for the IIfx, with
it's external black SCSI terminator) ship with terminators installed on
unused ports. Lacking one makes absolutely no difference, as well, having
used many macs, and many, many SCSI devices.
The slow SCSI busses and
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