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manual abstract
3 Accessories
3.1 SCSI Bus Termination
A SCSI bus is composed of a daisy chain of SCSI devices connected via SCSI cables. SCSI
buses need to be electrically terminated at each end of the SCSI bus chain, and only at the
ends. This termination is needed for proper operation of the bus. See Table 1 for details on the
termination types needed with each SCSI bus type.
Because SCSI buses are connected in a daisy chain fashion, SCSI devices normally have two
SCSI connectors to accommodate connections in each direction on the bus chain. If the device
is at the end of the chain it must be electrically terminated by either physically plugging a
terminator into the unused connector or it may be auto-terminated in the device (circuitry which
detects that the connector is not connected to another device).
3.1.1 HBA Auto-Termination
Typically HBAs are designed to connect at the end of a bus with a single connector. When an
HBA has two connectors they typically consist of an “internal” connector for connection to a
system-internal bus, and an “external” connector to connect to an external bus. If an HBA is
located at the end of a bus, it must be terminated internally onboard the HBA, either through
auto-termination circuitry or via manual switches or jumpers.
HBA auto-termination generally operates by sensing a pin on the internal and external
connectors of the HBA port to see if a device is connected. If both connectors have devices
connected6 the HBA will disable its on-board termination circuits; otherwise the on-board
termination is enabled. The auto-termination mechanism can also be affected by jumpers on the
card or programmatic7 controls.
HBA auto-termination works well as long as the HBA is at the end of a bus, or if it’s connected
between an internal and external bus. If an HBA is connected via a V-cable8 in the middle of a
bus the auto-termination won’t work as required. It must be disabled, and the on-board
termination forced off either programmatically7 or via jumpers.
3.1.2 Device Auto-Termination
The MSA30 MI auto-terminates its SCSI bus ports, sensing if a device is connected to it or not.
This provides an ease-of-use benefit over previous devices such as the DS2300, which required
physically plugging in a terminator in the empty SCSI port of a SCSI port pair.
3.1.3 Inline-Terminated Cables
Inline-terminated (ILT) cables8 violate the SCSI Specification at Ultra2 speeds and beyond. As a
result inline-terminated cables are not supported on Ultra2, Ultra160, and Ultra320 buses. On
Ultra or earlier configurations inline-terminated cables were used to allow a host at either end of
a shared (multi-initiator) bus to be disconnected without bringing down the other host or
stopping the applications accessing that bus. However, online reconnection of a host back to
the bus using ILT cables is not supported due to electrical issues that can arise while
reconnecting to a live bus.
6 Simultaneous connection of both the internal and external connectors of an HBA is not supported on HP-UX, which means that the auto-
termination circuitry will always enable termination unless it’s manually disabled. This is the desired behavior for HBAs at the ends of buses.
7 Software-controlled termination (programmatic control) is not supported on any of the currently available SCSI HBAs.
8 V-cables and ILT cables are not supported with Ultra2, Ultra160, or Ultra320 HBAs.
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