so I decided to start rolling that ball. I posted a lot about this topic back in the 90s in the comp.sys.mac.* hierarchy in the news groups, so at first, I'm just going to quote some of my old postings as I find them. After a week of desultory searching in Advanced Google Groups Search, I've found my first useful posting, quoted below Early SCSI History:
> Can someone concisely explain what the difference is?
> Can I use a drive that supports fast or wide or ultra with
> any old SCSI controller and just not get the maximal performance?
> Or, is it that the interfaces are incompatible?
> -- Al
This is probably explained better in the FAQ over on comp.periphs.scsi,
or that might be comp.periph.scsi. But here it goes:
[2012 Edit: comp.periphs.scsi FAQ, Part 1 of 2
[2012 Edit: comp.periphs.scsi FAQ, Part 2 of 2
In the beginning there was SCSI and it was pretty good and it maxed out
at transfer rates of about 3 MB/s. But that was the early 80's, so
it was faster than any hard drives worked anyway. There were
ambiguities in the IEEE standard as written, so SCSI from one company
wasn't necessarily compatible with SCSI from another company. This
problem mostly expressed itself in the PC world as little proprietary
SCSI controllers that were included with scanners or other peripherals
and that wouldn't work with any other SCSI devices. We had it good
in the Mac community when it came to SCSI, except that most of Apple's
implementations would do maybe 1.5 MB/s transfer rates.
SCSI was based on a 50 wire cable transferring 8 bits of data in
parallel. The other 42 wires are ground wires and various control
signals. The frequency of data over this cable is also part of
the standard but I don't remember the number. I think it was
10 MHz. [2012 Edit: It was 5 MHz.]
So it was decided to write a new SCSI standard that would vanquish the
ambiguity and while they were at it, they decided to add some
improvements, because 3 MB/s was getting pokey.
Thus was SCSI-2 born. By making use of synchronous transfers (or maybe
that was asynchronous transfer [2012 Edit: It was synchronous],
I get the two confused) SCSI-2 was
able to up the transfer rate to 5 MB/s using the same 8 bit parallelism
at the same frequency as the old SCSI standard. But the standards-makers
were not satisfied. So they added two more options to SCSI. It could
be Fast--operating at twice the normal frequency, and/or it could
be wide--transferring 16 bits of data in parallel instead of 8. And just
to help confuse things, they prescribed a new SCSI connector to replace
teh old Centronics 50--the High Density SCSI connector--often called
a SCSI-2 connector because it came out when the new standard was
And the standards-makers declared backward compatibility. The original
SCSI devices would work on the same bus as the SCSI-2 devices. And Fast
SCSI-2 devices would work with them all. But to take advantage of the
new features, both of the devices communicating must have the same
features. What this means in proactice is that the user would never
get performance better than what his SCSI controller supported and the
controller would have to slow down to the speed of whatever device it
was talking with at the moment.
Because Wide devices transfer 16 bits at a time, they need a different
cable. You must have some extra wires to carry those 8 extra bits.
So Wide devices usually go on their own 68 wire bus. But those wily
SCSI Standards-writers thought of everything. If you put an
adapter on a wide device and attach it to a narrow bus, or put
an adapter on a narrow device and attach it to a wide bus, the
controller and the device will negotiate their transfer method,
and agree to just use 8 bit transfers, so that you don't have the
other 8 bits getting lost in the ether somewhere.
Wide devices usually cost a little extra, so people usually don't
buy them and then handicap them by putting them on a narrow bus.
And putting a narrow device on a wide bus can decrease the overall
performance of the wide bus, as well as causing issues with SCSI
ID's over 7 (wide uses ID's 0-15), so it's best to avoid that too,
if you can.
A note--wherever I write "wide" I really mean Fast & Wide. I've
never seen one, but a SCSI drive that was simply Wide, would have
a maximum transfer rate of 10 MB/s, the same as a Fast narrow one.
It's the combination of Fast (twice the bus frequency) with wide
(twice as many bits in parallel on the bus) that gets the
20 MB/s theoretical transfer rate of Fast & Wide.
Still later a newer SCSI standard was born and it was called
Ultra-SCSI. I'm less familiar with this new stuff, but somehow
it upped the maximum transfer rate on the old familiar 50 wire
cable to 20 MB/s. Yet, it is backward compatible in the same way
that SCSI-2 and Fast SCSI-2 was. Just keep in mind, that if you
put an Ultra-SCSI drive on a plain SCSI-2 controller, your max performance
will still be 5 MB/s. And any old original SCSI devices on a fancy new
ultra-SCSI chain are still stuck at 3 MB/s. But an Ultra-SCSI
controller with Ultra-SCSI drives can get you 20 MB/s.
And following in tradition, there is wide ultra SCSI which does
twice as many bits at the same speed as ultra-SCSI, getting
a maximum theoretical performance of 40 MB/s.
So to summarize. There are devices built for 50 wire buses (50 pin
cables) which can all be connected to the same bus without adapters:
original SCSI 3 MB/s
SCSI-2 5 MB/s
Fast SCSI-2 10 MB/s
Ultra-SCSI 20 MB/s
In general only two devices on a bus talk to each other at any given
time. This is almost always (is always in any case the normal Mac
user will encounter) the controller (logic board or SCSI card) talking
with one of the devices on the chain. These two devices will com-
municate at the rate of the slower device.
And there are devices that are built for 68 wire buses:
Wide SCSI-2 10 MB/s (never seen one of these, don't think they
were ever built)
Fast & Wide SCSI-2 20 MB/s
Wide Ultra SCSI 40 MB/s
Wide devices can be adapted to narrow busses and narrow devices can be
adapted to wide busses, but when you do those devices will only give you
narow performance. And putting narrow devices on a wide bus may slow
down the performance of that bus with the wide devices.
Finally there are some 80 pin devices out there, but I'm not sure what
the deal is with them. I think they're built for hot pluggable
systems. And there are what are called Differential drives. These
use a differnet method of interpreting electrical signals as data,
so that they can better tolerate environments with a lot of
electrical noise and use longer cable runs. Differnetial drives
are not compatible with other SCSI drives and can cause damage if
attached to a regualr bus. There are some adapters for differential
drives, but they are so expensive, you are better off just getting
the correct drive in the first place.
Hope this helps. It's not concis...oh well.
Okay, that's rather incomplete because it's so out of date and not very concise. But I don't have time to write something short and to the point at the moment.
Oh, I also found this useful link: http://www.scsifaq.org/
I'm still looking for my old postings on configuring SCSI chains properly and the finer points of mixing wide and narrow drives. As I recall, those had some nice ASCII diagrams included.