35Chapter B - Hardware InstallationB.4 What you should know about SCSIIt is very important for you to observe the information and notes given in this section of theUserís Manual because it helps to ensure that the SCSI devices that are used in connectionwith the GDT Controllers are operated in a successful, long-lasting and trouble-free ma n-ner. In many cases this information is not only applicable to GDT Controllers but in generalto all those SCSI systems which, like the GDT Controllers, use Single Ended SCSI bus chan-nels. According to its definition, the SCSI bus provides access to several participants thatare physically connected through an appropriate SCSI bus cable. To achieve a sufficientlygood signal quality, it is not only recommended to use very good cables and connectors,but also to terminate both ends of the cable properly. For an unambiguous identificationon the bus, all participants have a unique number, the so-called SCSI-ID. Further details onthese topics can be found on the following pages. Please note that 98% of all SCSI-related problems are caused by bad SCSI cables, wrong SCSI bus termination andduplicate SCSI-Ds.Recently, strong efforts have been made to automate the setting of the SCSI bus termin a-tion and SCSI-ID on the SCSI bus. An appendix to the SCSI-3 specification with the titleSCAM (SCSI Configured AutoMatically) has been created. It includes a description of allthe functions necessary for building a SCAM compatible SCSI device or controller. Unfort u-nately, in real life SCAM is rather a definition than a useful help. Even worse, SCAM hasadded other problems and more confusion to the already difficult SCSI topic. As long as itis possible to buy and operate SCSI devices without the SCAM feature (99.9% of all cu r-rently available devices do not support SCAM), massive problems are very likely to occur.Therefore, the GDT controllers only rely on the well proven and standard method of settingSCSI-IDs and SCSI bus terminations and do not expect any further capabilities of the SCSIdevices.B.4.1 SCSI CablesThe quality and overall length of the cable, as well as the number and quality of the SCSIconnectors is very important for both internal and external SCSI cables. Generally, internalSCSI cables are 50 or 68 conductor flat ribbon cables. To connect external SCSI devices,round and shielded cables with appropriate connectors are typically used. The minimumcross section per line has been defined in the SCSI-3 specification as follows:• 50 conductor cables: minimum 28 AWG conductors and with• 68 conductor cables: minimum 30 AWG conductors.The typical impedance of a SCSI cable is 84 Ohm +/- 12 Ohms. The maximum difference inimpedance between two conductors of a SCSI cable must not exceed 12 Ohms. Externalround cables should have a SCSI-compliant placement of the inside conductors. Besidesthe cables, the right connectors for a cable are very important, too. We highly recommendto use highest quality connectors, only. The following table shows the maximum cablelengths allowed for a given transfer rate. Based on many years of SCSI experience, thelengths we recommend are in some cases shorter than theoretically possible. The inform a-tion in the table refer to one SCSI channel und represent the overall length of the cable,including internal and external parts.
