Article about SCSI standards. Published in Atari Magasinet #1, 1998 (swedish magazine). Translated into english by the author.


Find your way in the SCSI-jungle

Many of us use SCSI hard disks with our machines, but without the slightest idea about what it it or how it SHOULD be connected etc. Some go by the saying: "no pain - no gain", something that really isn't that funny all the time...

SCSI, pronounced "skassi", is an abbreviation of Small Computer System Interface. It's used mostly for hard disks, backup units, scanners, CD-ROM's and CD-writers - but it can also be used for many other applications, like modems and printers. For us Atarians, it's mostly the first ones that are interesting, since there aren't any drivers (to my knowledge) for the more odd SCSI applications.

SCSI is a standard. Weeeel......, is it or isn't it? When you check up on the facts you have the risk to get more confused than ever before. Well - judge for yourselves - here is a small line-up of the available standards.

SCSI-1
In the first version of SCSI, an 8-bit bus is used to transfer data at up to 5MB/s. With SCSI-1, it's possible to connect 7 SCSI-units and use a cable length at a maximum of 6 meters.

SCSI-2
SCSI-2 is a development of SCSI-1, with more commands and functions. Support for a quicker data transfer is implemented by the Wide and Fast standards.

Wide SCSI-2
With SCSI-1, it's possible to transfer 8 bits at a time on the bus, but with Wide SCSI-2, the bus width is expanded to 16 bits (which means higher transfer rate). You can also connect 15 units to a Wide SCSI-2 interface.

Fast SCSI-2
SCSI-2 can use standard asyncronous transfer at 5MHz or Fast asyncronous transfer at 10MHz. By speeding up the bus this way, the transfer rate can reach 10MB/s.

Fast and Wide SCSI-2
By combining the two techniques, the data transfer rate can be increased even more and reach up to 20MB/s.

Ultra Wide SCSI-2
With the Ultra technique, the bus speed has been increased from 10MHz to 20MHz, as well as using the 16-bit bus - it's possible to reach speeds of up to 40MB/s.

SCSI-3
People are actually calling all SCSI Wide, Fast and Ultra Wide for SCSI-3, which is wrong! With the real SCSI-3 standard (which isn't available yet), it will be possible to have 32 units on the same interface on a 16-bit bus with even hogher bus speeds than at present.

TERMINATION
The SCSI-bus must be terminated in both ends to work properly. If the ends aren't terminated, errors can occur in the data - especially when loading the bus with much information and if long cables are used. If you have two or more SCSI-units doesn't matter - only the first and last should be terminated. In Atari surroundings, the first unit is always terminated (since it's the interface), so only the last unit should have a termination. When it comes to PC's, it's a little different, since you have an interface card with options to use both external and internal units - but this is an Atari magazine, so we don't care about their problems... >;) If many of the connected units are terminated, you will get problems, since the terminators are mostly resistor packs which load the bus - and if that load gets too high, the data won't have a chance to get through...

There are different ways to terminate a unit. On some units, you disable or enable the termination with a jumper, while on others you need to remove the termination resistors completely. If the unit is located in an external box, the unit itself should never be terminated - it's better to use an external termination plug instead - that way you'll never have the risk of having more than one termination.

ACTIVE AND PASSIVE TERMINATION
In passive terminators, there are only a resistor pack and nothing else. This type of termination is mostly used on SCSI-1 units. Active terminators use the +5V from the SCSI-bus and use an active component (voltage regulator of some sort) to get the correct impedance in the cable. This is a much safer termination for the SCSI-bus and must absolutely be used to obtain really high transfer speeds.

CONNECTIONS
Internal SCSI-1 and SCSI-2 units use a 50-pin flat ribbon cable connector, while internal Wide units use a special 68-pin D-type. External connections are normally a 25-pin D-type and a 50-pin Centronics connector for SCSI-1, while Fast SCSI-2 uses a 50-pin high density D-type or a 50-pin mini Centronics. Wide SCSI-2 uses 68-pin connectors instead - to be able to transfer the wider data bus. A mixture of SCSI-1 and SCSI-2 is rather common, since the connections are the same in the hard disks, so using a 50-pin HD D-type to 50-pin Centronics cable is very common - both among Falcon- and PC-users.

WHAT DOES IT ALL ADD UP TO?
All the fancy stuff about high transfer rates etc doesn't really apply to us Atarians, since the Falcon's SCSI-bus is limited to 2MB/s and since the TT uses the same SCSI controller chip, these 2MB/s should be the limit for those too. Since those are the fastest machines around (among those I have a reasonable chance to test - Hades and other clones are probably faster), the question is if the Falcon's SCSI-2 connector isn't mostly used to impress people with saying "it's got SCSI-2". I suspect the main reason to use this type of connector was to save up space - keep in mind that the SCSI-2 standard has a transfer rate of up to 10MB/s and that even the SCSI-1 standard outperforms the Falcon's 2MB/s.

Even so - this speed is enough for HD recording and in Cubase Audio it's possible to replay 8 audio tracks simultaneously, so you actually dont need more than that - so why complain? You can even use a Wide SCSI-2 disk with the Falcon (using an adapter) - I've tried this myself without any problems in Cubase Audio!

Standard or no standard, but it seems like the various SCSI-standards are rather comptible with each other after all, even though they're rather unlike each other. Always be careful with how you terminate the system though - and if you get problems in spite of correctly mad etermination, it's a good idea to try active termination instead of passive. Oh! By the way - did I mention that you should never ever use anything else but SCSI-cables? I know of people who tried to use parallel cables instead, since they looked similar - and managed to destroy both the controller and the hard disk in the process - "no pain, no gain" or? Please - no flat ribbon cables outside of boxes - then you'll be asking for trouble.

Relax and get the right connections!
Claes
claes@holmerup.com
www.holmerup.com