OR
site rtfm.mit.edu in directory pub/usenet-by-group/comp.periphs.scsi/ as files comp.periphs.scsi_FAQ_part_*
OR
via World Wide Web (WWW):
http://www.cis.ohio-state.edu/hypertext/faq/usenet/scsi-faq/
OR
(an old version but in HTML format)
--/* Gary A. Field - WA1GRC, Wang Labs M/S 019-72B, 1 Industrial Ave
Lowell, MA 01851-5161, (508) 967-2514, email: garyf@wiis.wang.com, EST5EDT
Keyboard Defective: Press F1 to continue */
SCSI stands for Small Computer Systems Interface. It's a standard for connecting peripherals to your computer via a standard hardware interface, which uses standard SCSI commands. The SCSI standard can be divided into SCSI (SCSI1) and SCSI2 (SCSI wide and SCSI wide and fast). SCSI2 is the most recent version of the SCSI command specification and allows for scanners, hard disk drives, CD-ROM players, tapes [and many other devices] to connect.
SCSI is becoming a popular standard. More and more computers use it daily. (e.g. ATARI Falcon and TT, expensive MS-PC's, Amiga, Apples and many others)
One confusing thing about SCSI is what the SCSI bus is supposed to look like, and how devices should be placed on the bus.
The SCSI bus MUST run continuously from one device to another, like this:
DEVICE A --------- DEVICE B --------- DEVICE C -------- DEVICE D
The devices on the SCSI bus should have at least 4 to 6 inches of cable between devices. This is to satisfy the SCSI-2 requirement that "stubs" be placed at least .1 meters apart. Some devices that have a lot of internal wiring between the connector and the SCSI chip can look like a "stub" or bus discontinuity. The reason for all these requirements is that a SCSI bus is really 18 "transmission lines" in the wave theory sense. A pulse propagating along it will "reflect" from any part of the transmission that is different from the rest of it. These relections add and subtract in odd combinations and cause the original pulse to be distorted and corrupted. The terminators"absorb" the energy from the pulses and prevent relections from the ends of the bus. They do this because they (hopefully) have the same impedance as the rest of the transmission line.
The SCSI bus must not have any "Y" shape cabling. For example, setting up a cable that looks like this is NOT allowed:
DEVICE B
\
\
\
>------------- DEVICE C ----------- DEVICE D
/
/
/
DEVICE A
Termination must be present at two and ONLY two positions on the SCSI bus, at the beginning of the SCSI bus, and at the end of the SCSI bus. There must be no more than two, and no less than two, terminators on the bus.
Termination must occur within 4 inches (.1 meter) of the ends of the SCSI bus.
The following ARE acceptable:
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A Unconnected Unconnected Unconnected DEVICE B DEVICE C
Terminated (adapter-Terminated)
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A Unconnected DEVICE B Unconnected Unconnected DEVICE C
Terminated (adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
Terminated DEVICE A DEVICE B Unconnected Unconnected DEVICE C
(adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
DEVICE A DEVICE B DEVICE C Unconnected Unconnected Unconnected
Terminated (adapter) Terminated
+------------+----------+-----------+-----------+---------+
| | | | | |
Unconnected DEVICE A DEVICE B DEVICE C Unconnected Unconnected
Terminated (adapter) Terminated
The placement of the SCSI adapter card can be on the end, at the beginning, or somewhere in the middle of the SCSI bus. Quite frankly, placement of the controller card isn't special. The adapter card is just another device on the SCSI bus. As long as the rules above and in other sections of this FAQ are followed, there should be no problem placing the adapter card anywhere on the SCSI bus.
However, if you place the adapter card somewhere in the middle of the SCSI bus, you must be sure to disable termination on the adapter card. As noted previously, a SCSI device is only allowed to have terminations if it's at the end of the bus. Only two terminators are allowed to terminate the SCSI bus, one at each end.
One last note: It doesn't make any difference where each SCSI ID is placed along the bus. It only matters that no two devices have the same ID. Don't forget that the adapter has an ID too. (Usually ID 7).
Yes, two (or more) systems can be on the same scsi bus as scsi disk and tape drives. As long as the scsi requirements are met - cable lengths, termination and type - the devices can share the scsi bus.
The question should be - Are there any O/S' that will allow the sharing of file systems? It would not make sense for two hosts to go about treating shared disks as if they owned the device. Data would be destroyed pretty quickly.
On the issue of tape devices, however, O/S' tend to give exclusive usage to an application. In this way, tape drives can be shared much more easily.
Disks can be best shared by having two (or more) partitions on a disk. Each host "owning" its own file system.
Several articles lately have cited the importance of SCSI-2-compliant cables when cabling SCSI bus subsystems. Perhaps the most accurate and technically detailed one was published in Computer Technology Review in March (Volume XIII, No. 3. PP. 6). In short, it explains the double-clocking mechanism that can occur due to cables whose impedance falls below the 90-Ohm SCSI-2 spec. Steep edge speeds on the REQ and ACK lines of the SCSI bus exacerbate the problem, but non-compliant cables are the root cause. Both LAN TIMES in the US (5/24/93, page 115) and CT Magazine in Germany (7/93, page 18) cite this cable problem.
In an extensive survey of cables available in the US and Europe, we found that more than half of the cables available have single-ended impedances in the 65 to 80 Ohm range -- below the 90 to 132 Ohms specified in the SCSI-2 spec. It seems that some (not all) cable vendors do not understand the specification, describing their cables as SCSI-2 compliant when they are not. A common misconception is that SCSI-2 means a high-density connector. In fact, there are several connector options. I have published a technical bulletin that summarizes the critical requirements (TB 001, April 1993). An artifact of its faster design left the AHA-1540C with faster edge-speeds than its predecessor, the AHA-1540B. As I have said, this can exacerbate the effect of bad cables. This explains why some users could get their AHA-1540B to work when an early AHA-1540C might not. Essentially, the 1540B was more forgiving than the early 1540Cs. Good cables fixed the problem, but unfortunately for the user, good cables are hard to find.
After surveying the cable market and many of our customers, we decided that bad cables were going to be here for a while, and we had to make the 1540C as forgiving as the 1540B was. At the end of April we made a change to the AHA-1540C that involved using a passive filter to reduce the slew rate of the ACK line, the signal that the host adapter drives during normal data transfers. Extensive testing with many intentionally illegal configurations confirms that we succeeded. Prior to release, we tested the AHA-1540C with over 200 peripherals, systems and demanding software programs with no failures. Then, a second team retested the AHA-1540C across a wild combination of temperatures, humidities and other stresses. This testing gives me confidence that the AHA-1540 line continues to serve as the gold standard for SCSI compatibility.
The AHA-1542A is obsolete and no longer supported by Adaptec. They stopped providing firmware upgrades at some level prior to the equivalence to the 3.10 level of the AHA-1542B firmware. I am not sure just where though. The present latest AHA-1542B firmware is version 3.20, and supports drives up to 8GB under MS-DOS.
The 1542C is an an updated model which replaces the 1542B. The 1542C features jumperless setup, having only 8 DIP switches. All other configuration options are set using the 1542C's built-in BIOS configuration utility. Configurable features not found on the 1542B are:
The 1542CF includes all of the 1542C features, and adds "Fast" SCSI operation, providing SCSI data rates of up to 10MB/sec (compared with an upper limit of 5MB/sec on the 1542C). This is unrelated to the host DMA rate. It also has a software-configurable address for the floppy controller and a "self-healing" fuse for termination power.
ftp.psg.com:~/pub/adaptec/...
SCSICNTL.EXE.Z adse.dd adse.dd.readme list os2drv.zip scsi_drv.Z scsi_drv.readm update.pkg.Z
You can get the ASPI specs from Adaptec's Bulletin Board (408)945-7727.
[editor] You can also get ASPI spec's from Adaptec's WWW server.
As I promised I am posting the summary of what I learned about 128mb optical drives through many kind replies and some effort on my part. The purpose of this informal survey was to aid people (starting from myself) in deciding on which 128mb optical drive to buy.
When I posted my questions, it was done only on comp.mac.sys.hardware and forgot to do the same also on comp.arch.storage and comp.periphs.scsi where are less traffic than c.m.s.h. However, as a Macintosh owner myself, this survey was biased toward the Mac world and the mail order houses mentioned specializes in Mac-related products, although the below mentioned optical drives might be usable also with non-Mac platforms (Sun, NeXT, PC-compatibles).
My questions were:
In general, these days, some magneto-optical (MO) drives seem to be almost as fast as (if not faster than) ordinary hard drives (HD). The access time of fastest 128MB MO drives (around 30ms) are slower than average HD's access time (15ms) but the transfer rate seems to be about the same (764KBytes/sec) or not much slower. The advantages of the MO drives over the HDs are that your storage space is almost limitless, expandable at a relatively cheap price ($40/120MB = 34 cents/MB) compared to $1/MB rate of HDs or that of SyQuest drives, and the life of the media is very long (they say it's 30 years or rewritable 100,000 times.)
Fijitsu 128 REM Portable: At this moment, to my knowledge, 128 MByte optical drives based on Fujitsu mechanism seems to be the fastest, roughly having average seek time of 30ms and average transfer rate of 768KBytes/sec. Another good thing about this Fujitsu drive is that it is more compact in size than previous 128mb optical drives, ie. "portable". I don't know how Fujitsu mechanism (FM) is different from Epson mechanism (EM) and how FM provides a similar performance at a cheaper price in a smaller frame. Maybe using split-head implementation to make the read-write head lighter? Could anybody post info on this? One person tells me that the eject mechanism is too strong, sometimes shooting the cartridge out making them land on the floor. He says Fujitsu told him that the FM's coming out in April will have gentler eject.
Epson: The next fastest (or maybe just about the same speed) are Epson mechanism (EM) drives, having average access time of 34ms and transfer rate of 768KB/sec. These achieve faster speed compared to other old mechanisms by having a higher rpm (3600rpm vs. past 2400 rpm).
Slower ones: Other mechanisms (Sony, Panasonic, etc.) seem to have been dominating the optical drive market before FM and EM's advent. These have a typical access rate of ~45ms. I don't know if now there are new implementations that make them perform better then FM and EM. Maybe someone can tell us.
Noise Level: One thing to consider might be noise of the drive. Different mechanisms may have typical noise level, but one thing sure is that different resellers/companys' drive's noise level differs even for the same drive mechanism, eg, Sony. It looks like different casing produces different noise levels? (Could someone confirm/disconfirm this aspect?) Base on the report in Nov '92 issue of MacWorld, the noise level of MacDirect, MacProduct and DGR 128mb MO drives seem to be OK or quite quiet.
This issue of MacWorld deals with removable media drives (optical drives of various capacity, SyQuest, Bernoulli and Flopticals) and you can get some idea on what the differences among different drive mechanisms are.
Formatting Software: Another thing to consider is what kind of media formatting software you will use. All companys (or mail order places) seemed to provide for free formatting program with their drives. I don't have the details on this. But an inefficient formatting can result in slow drive performance. The most popular one used to be FWB's Hard Disk Toolkit but Anubis (advertised to improve performance up to 35% [compared to what?]) is beginning to be used also. I don't know if all formatting program and the drive hardware allows to have read and write verify off but by having these turned off you can obtain significant speed boost at the risk of less secure data transfer. MacWorld's report warned that drives from some companys don't let you turn on/off the verify. In the worst case, some come with verify off and no option to toggle it back to ON.
256mb MO drives: In general these have better transfer rate (1.23MB/sec) and a little slower access time (35ms). I feel that this capacity will soon be the next standard. These drives are able to also read/write 128mb cartridges and 256mb will soon be new ANSI and ISO standard. I once heard from a saleperson at a mail order place that these are not reliable yet and he saw many they sold came back with complaints. This may be a non-general instance on a typical drive mechanism (seems to be MOST mechanism). Personally, I feel 128mb is accomodating enough for personal usage at home unless you are dealing with very large data files (eg, large graphic images).