Archived - PowerBook: SCSI Bus, Termination, and Termination Power

PowerBook Termination and SCSI Disk Mode

On most PowerBook computers, termination is supplied by the internal hard drive termination. However, PowerBooks using IDE (ATA) hard drives lack SCSI termination. This includes the PowerBook 1400 series and PowerBook 3400 series (see note).

PowerBook computers depend on the internal hard drive to supply termination, so that they are properly terminated when placed in SCSI disk mode. In SCSI disk mode, the PowerBook is just another hard drive in the SCSI chain, because all other subsystems have been shut down.

With the introduction of IDE (ATA) specification internal hard drives in PowerBook computers it is necessary to supply double-termination to the SCSI bus. When connecting a device with internal termination an extra external terminator should be placed between the cable and the device. If the device lacks internal termination a total of two external terminators are required. One should be placed between the cable and the device, and the second terminator should be applied to the second port on the device.

*Double-termination does not apply to Disk Mode or Target Mode applications.

Because the PowerBook is internally terminated by the hard drive, it must always be at the end of the SCSI chain. If you have two PowerBook computers and other devices on the chain, one PowerBook must be the very last device in the SCSI chain (in SCSI disk mode) and the second PowerBook must be at the beginning of the chain. It is not possible to have more than one PowerBook on a single SCSI bus in SCSI disk mode.

PowerBook Termination Power

Some all-in-one PowerBook computers and the Duo series supply termination power. Those models that do not supply termination power rely on external devices to provide termination power on the bus. Because termination power is not supplied by these PowerBook computers, Apple advises that you do not have powered-off devices on the bus.

PowerBook Duo computers, the PowerBook G3, PowerBook G3 Series, PowerBook 3400 and PowerBook 2400 do not rely on an external device to provide termination power. The DuoDock provides termination power to the bus. They are actively terminated, which means they provide termination power and have the best architecture for termination of any other device on the bus.

SCSI System Cable and Adapter

Apple's SCSI cables are built to tight specifications to minimize electrical noise on the SCSI bus. The wires are laid out to provide minimal signal degradation. Poor quality cable construction may lead to intermittent SCSI operation and potential system crashes.

HDI-30 Adapter SCSI Cable (socket) & HDI-30 SCSI Cable (plug)
* Material: 30 conductors 26 AWG 90% braid plus foil shield.

Note

For the PowerBook 1400, it is necessary to supply double-termination to the SCSI bus. When connecting a device with internal termination an extra external terminator should be placed between the cable and the device. If the device lacks internal termination a total of two external terminators are required. One should be placed between the cable and the device, and the second terminator should be applied to the second port on the device.

*Double-termination does not apply to Disk Mode or Target Mode applications.

For electrical, mechanical, and shielding specifications, see the following Tech Info Library article:

Article 5475: "SCSI: Cable Specifications"

For more information on connecting SCSI devices, please see the following Tech Info Library articles:

Article 13611: "PowerBook: Using SCSI Devices"
Article 9387: "Macintosh: Connecting SCSI Devices"