SCSI crash course

[kallikru]

I see a lot of people here having trouble with SCSI cards and devices, so why not do a little crash course on SCSI:

Some facts:

A SCSI (host)adaptor is a device controller much like an IDE controller found in every modern PC today. It has the abillity to control datastreams flowing to and from a number of devices, provided these are equipped with SCSI-interfaces/connectors.

Whereas a normal IDE controller handles harddrives, CD-ROMs, DVD-drives and other storage devices, SCSI controllers may also control other devices like scanners and CCD-cameras.

You average IDE controller will handle up to 2 devices (master/slave), and since a PC is normally equipped with a primary and a secondary IDE controller, this sums up to a max of 4 devices.

A SCSI controller will handle up to 7 or 15 devices, depending on the type of controller. With a SCSI controller there is no such thing as master/slave - instead SCSI-IDs are used, numbering from 0 to 7 (15) - The adapter usually has the ID #7 reserved for itself.
Another notable difference between IDE and SCSI controllers is that SCSI contr. most often have an external connector allowing the user to connect external devices to the controller. Some controllers have several different internal connectors to allow different types of SCSI devices to be connected (SCSI, SCSI-2, fast-SCSI, ultrawide-SCSI, LVD-drives, just to mention a few. I suggest you read here for further information: http://www.scsifaq.org/ ).

All these devices connected to the SCSI controller - internal as well as external - comprise the so-called SCSI-chain.

An IDE connector is 40 pins wide, SCSI connectors can be 50 pins, 68 pins or 80 pins. The ribbon cables are accordingly wider (except for ultra-cables). External cables are round and connectors are usually 50- or 68-pins high density (HD) - and quite expensive....some devices (like my old external ZIP 100 drive) require the old-type (or Mac?) 25 pin cable.

SCSI has throughout the years been known for its abillity to transfer data faster than IDE (due to a wider data-bus), but this speed difference gap has diminished noticable in recent years with the introduction of ATA66 and ATA100 in the IDE-world. And the introduction of USB 2.0 sends a third player onto the field.


When using SCSI there are some things/rules you should be aware of:

- All devices connected to a SCSI controller must have unique SCSI-IDs assigned. This goes for internal as well as external devices!

- Each end (internal <-> external) of a SCSI-chain MUST be properly terminated. Most modern SCSI devices have builtin termination, but this option must be enabled in order to work. External devices usually have 2 (HD) connectors - one FROM the previous device in the chain and one TO the next device (if any - which connector you use is normally up to you) - and a switch for termination.
Internal devices only have one connector allowing for connection with a ribbon cable. If the device you attach is the last one on the internal chain, you must connect this device to the LAST connector on the ribbon cable in order to obtain termination of the chain.

- In order to be able to boot from a SCSI device, the device must be defined (in the adaptor settings = SCSI-bios) as bootable. Also, this may have to be defined in the motherboard's bios.

- Some older SCSI harddrives require that a startup signal is sent to them during boot, to set them spinning. This may also be set up in the adaptor settings.

- Some external SCSI devices (fx. scanners) may be switched off. In order to be able to use these devices they must be detected by the adaptor during boot, which is why SCSI devices should ALWAYS be switched on before the PC is powered up. Sometimes a re-scan (with a piece of software like Adaptec's EZ-SCSI) from inside Windows will find such devices later, when they have been switched on, but personally I don't rely on that.
Internal devices naturally don't have any such power-on problems.

Once SCSI devices like harddrives, CD-ROMs ect. have been properly installed (incl. drivers) and are detected during boot, they can be handled like any other device in the system, and don't require any special software to work.


Well, quite a bit of writing here - I could go on, but that would probably become too boring for most. This is, after all, just a crash course intended to give an overview and maybe prevent the most common erros encountered when using SCSI.

If you have questions, feel free to post them here at VDr.

------------------
Karl, Denmark
---------
"..and may The Force be with you too..."

[This message has been edited by kallikru (edited 05-24-2001).]

[greengoose1]

kallikru, good write up. Which is better from an operational standpoint?

------------------
Seek knowledge and all else will follow


Please post back results - Press Ctl D to bookmark

Information

[kallikru]

Depends on what you want to use it for. When talking of speedy devices, SCSI is still in front. And if you want to hook up more devices that the 4 you're offered by many mobos with standard IDE controllers, SCSI may be the answer. If you have to use your machine for some data-intensive computations with lots of disk access, I would also recommend SCSI.

A thing I didn't mention above is the fact that (SCSI)bus-traffic and device prioritizing is handled by the controller (works by some sort of internal system comparable to IRQs) and therefore doesn't require CPU ressources. With a harddrive and a burner connected to a SCSI controller I can burn CDs from the harddrive AND let the machine perform some other ressource-heavy task at the same time - no problem.

Initially I started to use it because I have some devices (fx. CCD-camera for my telescope) that require SCSI. I tried to live with a serial connection for a couple of years, but was often nearly freezing my a$$ off (middle of winter= -10 C) while waiting for pictures to download.

With time I've grown pretty fond of SCSI and I have gathered quite a buch of devices hooked up to my systems (external burner, ZIP drive, tape backup drive) which I would like to be able to share between machines.
This requires a restart, of course, but is easily done by simply switching to another cable at the first device of the chain.

To me SCSI drives seem more stable, especially hard drives compared to IDE drives. True, you pay a lot more for SCSI drives, but so far it's been worth it for me: during the last 5-6 years I've only managed to crash one SCSI disk while 3-4 IDE drives have made it to the local electronics graveyard. So you get what you pay for.

I still DO use IDE disks though (they're nicely cheap and pretty large), but usually in a raid 1 configuration - I simply don't really trust them anymore.

------------------
Karl, Denmark
---------
"..and may The Force be with you too..."

[slipe]

You can usually activate a SCSI device that was not on at boot by highlighting the SCSI card in device manager and clicking refresh. I would think it would work transferring devices between computers. Works with Win 98/ME. You can also do it by highlighting “Computer” in device manager and clicking the refresh button but it refreshes everything on the system – might as well reboot.

Raid 1 is smart. Most people who use Raid go for 0 for the performance boost but double their data loss probability.

[kallikru]

Good point. I have noticed that SCSI-devices (and others, I guess..) are detected twice during startup: The first time when the SCSI controller is initialized and the second time during Windows startup when the drive lights on devices are shortly lit up. So it seems quite logical that one should be able to use the refresh method decribed to detect devices at a later time.

------------------
Karl, Denmark
---------
"..and may The Force be with you too..."

[zorroonecrow]

when i put in a second device it goes on which side of belt? near the harddrive?then that means its terminated? i have two that were taken out of 386 that were drive c. do i have to move a jumper.

i can never figure out jumpers for ide so ive never had a second ide drive.

what to i type to back up files from drive c.

thanks

------------------
o4 breathalyzer is a safer lower drunk driving test in some states

[kallikru]

Since each device has a unique ID#, you can put it on any side you like as long as the chain is still terminated, either by builtin auto-termination or by the last connector on the cable being used.

Usually there's a description on the device explaining how jumpers must be set to obtain specific IDs. Try connecting the devices one by one - the SCSI-init. will tell you what ID# each device is set for. If any devices use the same ID# you must change the jumper setting on one of these devices in order to assign another (unused) ID# to one of the devices.

------------------
Karl, Denmark
---------
"..and may The Force be with you too..."