The Labs.Com System Lab SCSI Lab

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Small Computer Systems Interface

SCSI (Small Computer Systems Interface) is the standard to attach hard-disks, and other peripherials.

1. Types
2. Termination
3. Resources

There are several types of SCSI available:

Name Bits Transfer Rate Data Rate SCSI-1 8 5 MHz 5MB/s SCSI-2 8 10 MHz 10MB/s WIDE-SCSI 16 10 MHz 20MB/s ULTRA-SCSI 8 & 16 20 MHz 20MB/s & 40MB/s ULTRA2-SCSI 16 40 MHz 80MB/s ULTRA3-SCSI 16 80 MHz *) 160MB/s

*) it's still 40MHz but rising and falling slope are used which results in comperable 80MHz

It has to be noted that rarely disk-drives really go to that data-rate, mostly 60% or even less, unless you use a RAID controller.

SCSI vs IDE Good overview of SCSI and IDE interfaces

SCSI Lab 2. Termination

Termination is required on both ends of your SCSI-chain. Now, most setups aren't so easy to overview, therefore make a drawing of your system.

We assume you have a UltraWide Controller like Adaptec 2940UW (which is very common, but not the best choice).

The controller can have "Auto - Termination", then the controller detects if its the end of the chain. If you mix narrow and wide with it, you likely have to watch this closely and turn off "Auto - Termination" (more below).

You attached several wide and narrow devices either externally or internally:

• only wide is used, last device terminated
• wide internal and narrow is used, both ends are terminated
• internal wide used last device terminated, extern wide used too, last device terminated.
• trying to connect all three, no proper termination possible therefore do not connect all three connectors.
• you can leave some connectors of your internal cable open, but the last physical connector must be terminated.
• doesn't matter if the last device is terminated, if it's not really physically the last, don't do it.

A possible solution is a dual-channel 3940UW or alike controller, then you have two independent SCSI-buses. For performance reasons you likely may choose two 2940UW instead a dual-channel 3940UW. Symbios SCSI controllers are recommended over Adaptec, since Adaptect doesn't fully agreed on SCSI defintions which NT doesn't care, but UNIX as Linux, FreeBSD etc you might not have a stable system, therefore despite its assumed preference, choose Symbios or NCR UW-SCSI controller.

Below a realistic setup:

In this setup it's most important you leave the intern 50 pin SCSI open. You cannot connect to all three connectors devices! We recommend to have a the intern SCSI chain the last device as wide and termination ON there. Externally either wide & narrow devices mixed, or just narrow. Assign high SCSI-IDs to wide devices, so the narrow devices use SCSI-ID 0-6. We assume the controller as SCSI-ID 7. The two narrow devices internally use adapaters, use those without termination. The #0 is the boot device (default), some controllers as 2940UW allow to assign boot device.

Finally, use the SCSI-BIOS Setup from Adaptec to limit the transfer rate of the SCSI-2 devices, and set the termination for the "outer" end of the SCSI-chain so to speak.

Again, the SCSI-chain goes "intern" and ends with the UW SCSI 9GB (just as example), and the other end of the chain ends "extern" with the the CD-ROM. Note, the physical chain is important, SCSI-ID do not matter at all in the consideration which devices needs to be terminated.

SCSI Lab 3. Resources

SCSI FAQ Very worthwhile infos

HardwareGroup: FAQ Misc hardware infos (incl. SCSI)

                                                                                                                                   

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