SCSI Bus Termination

Red Hat Cluster Suite: Configuring and Managing a Cluster
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# D.3. SCSI Bus Termination

A SCSI bus is an electrical path between two terminators. A device (host bus adapter, RAID controller, or disk) attaches to a SCSI bus by a short stub, which is an unterminated bus segment that usually must be less than 0.1 meter in length.

Buses must have only two terminators located at opposing ends of the bus. Additional terminators, terminators that are not at the ends of the bus, or long stubs cause the bus to operate incorrectly. Termination for a SCSI bus can be provided by the devices connected to the bus or by external terminators, if the internal (onboard) device termination can be disabled.

Testing has shown that external termination on HBAs that run at speeds greater than 80MB/second does not work reliably.

When disconnecting a device from a single-initiator SCSI bus follow these guidelines:

- Unterminated SCSI cables must not be connected to an operational host bus adapter or storage device.

- Connector pins must not bend or touch an electrical conductor while the SCSI cable is disconnected.

- To disconnect a host bus adapter from a single-initiator bus, first disconnect the SCSI cable from the RAID controller and then from the adapter. This ensures that the RAID controller is not exposed to any erroneous input.

- Protect connector pins from electrostatic discharge while the SCSI cable is disconnected by wearing a grounded anti-static wrist guard and physically protecting the cable ends from contact with other objects.

- Do not remove a device that is currently participating in any SCSI bus transactions.

To enable or disable an adapter's internal termination, use the system BIOS utility. When the system boots, a message is displayed describing how to start the utility. For example, many utilities prompt users to press [Ctrl]-[A]. Follow the prompts for setting the termination. At this point, it is also possible to set the SCSI identification number, as needed, and disable SCSI bus resets. Refer to Section D.5 SCSI Identification Numbers for more information.

To set storage enclosure and RAID controller termination, refer to the vendor documentation.

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