SCSI BUS & TERMINATION REQUIREMENTS ___________________________________ IBM OEM STORAGE PRODUCTS DATA SHEET GEN001 _________________________________________________________________________ ALL FAST-SCSI DISK DRIVES Outline This application note describes the necessary physical bus techniques for using Fast-SCSI drives such as the IBM 0662, 0663, and 0664 ranges. Some customers have reported problems in workstation applications, often in machines which have Fast-SCSI controllers such as the Sun SPARC and others. In many cases, these have turned out to be errors in cabling or termination violating the ANSI SCSI specification requirements. Fast-SCSI One of the ANSI SCSI-2 specification features is a 10 MHz option--this is Fast-SCSI. With an 8-bit bus this can transfer 10 MB/S (50-way cable), and with a 16-bit bus it will support 20 MB/S (68-way cable). Single-ended or differential bus configurations can be used. Single-ended signals are relative to ground while differential signals are transferred as a pair of signals of opposite polarity using differential drivers and receivers to achieve better noise immunity than single-ended. Single-ended and differential devices cannot be connected to the same bus, although the physical cables and connectors are the same. Physical Cable Limitations It is important to realize that cables carrying signals at these frequencies are acting as transmission lines, that is they have a characteristic impedance like other radio-frequency cables. In addition, if the cables are of the wrong impedance, or are terminated incorrectly, signal reflections can occur which will cause false data clocking and hence bus errors. Fast-SCSI busses MUST use either twisted-pair "flat" cable or twisted and shielded external cable. Short lengths of the cheap flat ribbon cable can be used, but are not recommended except in minimal systems. Single ended Fast-SCSI busses have a total length limitation of 3 meters. In addition, care must be taken to minimize stubs. Stubs are lengths of cable connected to the main bus in a "Y" or "T" fashion. Devices should instead be connected in a "V" configuration. Stubs are a common error in external SCSI boxes. The cable should run from one of the bulkhead connectors to the drive, and then back to the other bulkhead connector. A common error is to link the bulkhead connectors by a length of cable which then is connected by a stub to the drive(s). The ANSI SCSI committee recommend stubs be minimized, 5cms maximum length and at least 0.3 m apart. ---------------------------------------------------------------- Passive Terminators (S-E) The so-called "passive" older style terminators used 330 and 220 ohm resistor networks with the terminator power supplied directly from pin 26 of the 50-way cable. This design of terminator is designed ONLY FOR USE AT 5MHz. They are found in several forms: 1. "Inline" terminators used at the end of a cable to connect a device which does not have on-board terminators. 2. Bulkhead or external terminators--these may be found in either Centronics plug format or in 50 or 68-pin Micro-D designs. 3. On-board terminator R-packs fitted to disk drives, CD-ROM drives, etc. Active Terminators Active terminators contain 110 ohm resistor pull-up/termination resistors for each bus line, and also an active voltage regulator to supply 2.85 volts for these resistors. The regulator uses the 4-5 V terminator power line in the cable for input power. ****************************************************** Fast-SCSI devices MUST BE USED WITH ACTIVE TERMINATORS This is NOT optional!! ****************************************************** Active terminators are found in several forms-- 1. "Inline" terminators used at the end of a cable to connect a device which does not have on board terminators, eg, IBM 0664 drives. S-E examples: DataMate DM550-06-ADR and Amphenol 33770. These can be distinguished from the passive versions by the small box-shaped moulding on one of the long sides. This contains the regulator components. Inline terminators are only available for 50-pin cables. 2. Bulkhead or external terminators--these may be found in either Centronics plug format or in 50 and 68-pin Micro-D designs. Examples of S-E active versions are: Amphenol 22431 (Centronics plug) Amphenol 28705 & 16708 (50-pin Micro-D) Amphenol 16715 & 33321 (68-pin Micro-D) 3. FPT and Slick terminators--These are an active pull-up design originated by IBM, and are extremely effective in eliminating signal overshoot. They are available in Centronics and Micro-D format. Examples of S-E FPT terminators are: Aeronics FPT-18 (Centronics) DataMate Slick DMT2050-02-68SK (68-pin Micro-D) 4. On-board active terminators fitted to target devices, such as IBM 0662. These are usually selectable electrically by a jumper. Another jumper allows the drive to supply terminator power instead of (or as well as) the controller. -------------------------------------------------------------- Recommendations for Fast-SCSI Good Practice 1. In Fast-SCSI or mixed Fast/Standard configurations, MAKE SURE that the last device on each end of the cable has an ACTIVE TERMINATOR. This means that external devices such as CD-ROM and tape drives must have their on-board passive terminators removed, and an active terminator then fitted to the spare bulkhead connector on the last device on the bus. 2. In systems with an external SCSI port, if this is not connected to external devices, MAKE SURE there is an active terminator fitted to it. Often systems will work without one, but this contravenes the ANSI SCSI spec and WILL NOT run reliably in all environments. 3. In some systems with mixed Fast/Standard devices, particularly with external cables, it is often beneficial to supply terminator power to the last device on the external cable FROM THAT DEVICE. This improves term power decoupling and reduces noise. 4. Keep total cable length below 3 meters, avoid any stubs, and use good-quality twisted-pair flat or shielded cable. Twisted-pair cable should only be used inside enclosures and should not be laid onto metal chassis as this affects the impedance. All trademarks are acknowledged. This information is provided for the use of OEM Manufactures attaching IBM disk drives & is believed to be accurate but is supplied without guarantee.