Description

This item is from the PC Hardware FAQ, by Willie Lim and Ralph Valentino with numerous contributions by others. (v1.25).

3.12 How am I suppose to terminate the SCSI bus?

Some basic rules on termination:

1. The SCSI bus needs exactly two terminators, never more, never less.

2. Devices on the SCSI bus should form a single chain that can be traced from the device at one end to the device at the other. No 'T's are allowed; stub length should be kept as short as possible.

3. The device at each end of the (physical) SCSI bus must be terminated, all other devices must be unterminated.

4. All unused connectors must be placed _between_ the two terminated devices.

5. The host adapter (controller) is a SCSI device.

6. Host adapters may have both an internal and external connector; these are tied together internally and should be thought of as an "in" and "out" (though direction has no real meaning). If you have only internal or external devices, the host adapter is terminated otherwise it is not.

7. SCSI ID's are logical assignments and have nothing to do with where they go on the SCSI bus or if they should be terminated.

8. Just because your incorrectly terminated system happens to work now, don't count on it continuing to do so. Fix the termination.

Examples:

      internal      external           internal            external
    T------|-----|------T          T------|-------|-----|------|------T
 drive   drive   HA    cdrom      tape  unused unused   HA   drive  drive


        internal             external             external
     T------|-----T      T------|------T          T------T
   drive  drive   HA     HA    tape   cdrom       HA    cdrom

"T" = terminator   "|" = connector (no terminator)   "HA" = Host Adapter

 

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