R. R. "Dallas" Cook-Robinson originally posted this to the SuperMacsemail list. It is reprinted with his permission.
After lurking the SuperMacs list for a few months, I finally decided it was time to post a contribution of my own. I left my asbestos underwear at the cleaners, so be kind, would you?
I've seen a lot of issues discussed and a lot of opinions expressed (sometimes stated as fact, unfortunately), but nothing seems to have generated as much confusion as SCSI. SCSI isn't the black art it sometimes seems to be, but the various terms different people use for the same thing makes it tough to sort out. I hope the following lessens the confusion.
SCSI is a set of standards for parallel (multi-wire) data transfer among physically separate devices. The original standard (SCSI) has been rewritten twice to give us what is now called SCSI-1, SCSI-2, and SCSI-3. Each standard defines electrical and physical properties mostly of interest to electrical engineers, but people like simple names so various implementations acquired catchy titles which don't always make that much sense.
The original standard, now called SCSI-1, had only one flavor, so there is little confusion about it. It is the 8-bit wide, 5MBps (maximum) stuff most of us know as the standard Mac SCSI interface. It supports 8 SCSI IDs, 0-7, one of which must be a host computer. It's great advantage is the 20' (6m) maximum SCSI chain length it supports and the large number of devices available, particularly scanners and small removable media drives.
SCSI-2, Fast and Wide
SCSI-2 started the trend toward multiple implementations of the same standard - and multiple (and confusing!) names. SCSI-2 implementations are known as Fast because the maximum transfer rate was doubled to 10 MegaTransfers per second. A MegaTransfer is one million bus transfers without regard to bus width.
This was important, because SCSI-2 introduced bus widths wider than one byte to the SCSI standard. (While 32-bit wide transfers are a part of the standard, only 8- and 16-bit implementations have been marketed.) Thus, we get Fast SCSI, an 8-bit data interface with a maximum transfer rate of 10 MBps, and Wide SCSI, a 16-bit one with a maximum of 20 MBps - the same electrical characteristics and limitations, but twice the number of data lines for twice the throughput. Wide SCSI sometimes goes by the name Fast and Wide SCSI, but that usage is dying out.
The addition of the wider data path made it simple to add an address line to the standard, so all Wide SCSI implementations support 16 IDs (0-15). The SCSI-2 standard supports all the IDs, so Wide SCSI can support up to 15 devices plus a host.
The improved transfer rate came with the penalty of reduced SCSI chain length, limited to 3 meters (9.8'). It is important to understand that ribbon and simple twisted pair cables, while cheap, are not optimum transmission devices for high speed digital signals. These signal pulses bounce back along wires whenever they encounter any change in the construction of the wiring and generate crosstalk, ghost signals in adjacent wires. The longer the cable and the faster the pulse rate, the more these effects degrade the signal, placing limits on cable length and/or transfer speed.
SCSI-2 opted to reduce length to boost transfer speed.
SCSI-3, Ultra SCSI
By the time SCSI-3 came along, inexpensive electronics that could resolve faster pulses were practical in consumer gear. This let engineers double the internal clock rate of SCSI processors, and Ultra SCSI was born with 20 MegaTransfers per second performance. In theory, both narrow (8-bit) and wide (16-bit) Ultra SCSI interfaces are defined, but it is rare to see a device with narrow Ultra SCSI fitted to it. Thus, Ultra SCSI devices operating on a wide bus can do 40 MBps. If they are included on a narrow chain, they max out at 20 MBps, just like Wide SCSI.
Unfortunately, increasing the clock rate doesn't change the behavior of the cables, so Ultra SCSI is sensitive to cable length. 3 meter (9.8') Ultra SCSI chains will work reliably, but only when no more than 5 total devices (including the host) are attached. Limiting the chain length to 1.5 meters (4.9') allows 6-8 total devices on the chain. For reasons I won't go into here, 8 total devices is the practical limit for Ultra SCSI.
Ultra2, LVD SCSI
But there is more to the SCSI standards than discussed so far. One of the specs is the electrical transmission method that defines voltages and other characteristics on each wire in the interface. Without getting into a long discussion, suffice it to say other implementations that sacrificed cost and compatibility for longer cable lengths have been around since SCSI-2, but they don't apply to desktop systems most users will encounter.
However, under SCSI-3, a new, relatively inexpensive and completely compatible electrical standard that allowed long cable lengths was defined. This is Ultra2, also called LVD SCSI. By devoting two wires to each signal and only looking at the difference between them, the limitations of cable effects on signal integrity is greatly reduced. This doesn't double the numbers of wires in the cable, because conventional designs pair each signal wire with a common ground wire, but it does double the number of line drivers required, so it is more expensive.
Ultra2 takes advantage of this connection standard to produce a bus with a 40 MegaTransfer per second maximum transfer rate, a maximum cable length of 12 meters (29.4'), and support for 16 device IDs on the SCSI chain. Since it is invariably offered only in a wide version, the maximum transfer rate is 80 MBps.
To sum it all up, here's a list of the major characteristics of each SCSI flavor:
SCSI-3, Ultra SCSI(specs vary by cable length and bus width, but no separate naming conventions)
5 devices supported on <3 meter (9.8') cables, 6-8 devices supported on <1.5 Meter cables
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