dave@dms3b1.uucp (Dave Hanna) (11/02/90)
In article <304@rusux1.rus.uni-stuttgart.de> wschmidt@IAO.FhG.de (Wolfram Schmidt) writes: >In article <15476@cbmvax.commodore.com> andy@cbmvax.commodore.com (Andy Finkel) writes: >[...] >> >>1) You need to properly terminate the SCSI bus for more than one drive >> to work. (A SCSI bus needs to be terminated at each end, and no >> more than at each end. This may involve removing terminating resistors >> from the middle drives) >[...] > >So what happens if you have multiple drives, one internal and some external? >The bus is terminated at both ends. The host adapter is terminated, too. >Otherwise the manufacturer of the host adapter should have supplied a >terminator for the external port. At least my 2090 has terminators on board. >So when I soon connect an external SCSI tape drive the bus will have three >terminators. Any comments (especially form peolpe who design such things)? > >Wolfram > Ideally, the manufacturer _should_ provide a terminator at the external port, and some larger systems (e.g., Solbourne) do just that. The SCSI bus is terminated by a resistor pack built into a 50-pin Amphenol (SCSI) cable connector, which is plugged onto the vacant SCSI expansion connector. It makes properly terminating your expansion boxes easy. However, in most systems, there will be a controller card, with either a cable running to the internal drive and continuing on to the expansion connector, or one cable running to the internal drive and a second or an extension of the first running from the controller to the expansion connector. There are two SCSI devices on the bus - the internal drive, and the controller (The controller is considered another SCSI unit, just like a drive). There will be terminators on both of them. When you add expansion unit(s), you need to remove the terminator from which ever device (the drive or the controller) is in the electrical middle of the (now extended) bus. If the internal cable ran from the controller to the internal drive to the expansion connector, remove the terminator from the internal drive. If the cable runs from the controller directly to the expansion port, then the controller is in the electrical middle, and you remove its terminator. (hope this makes sense. I probably shouldn't post at 1:30 in the morning.) -- Dave Hanna, Infotouch Systems, Inc. | "Do or do not -- There is no try" P.O. Box 584, Bedford, TX 76095 | - Yoda (214) 358-4534 (817) 540-1524 | UUCP: ...!letni!dms3b1!dave |