Specifications

DV 15 - SCSI Termination Page: 3

There are two important points that one must be aware of when it comes to termination on the Macintosh PowerBook

computers. The first point is that termination on the PowerBook computers is supplied by the internal hard drive.

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. This is because

all other subsystems have been shut down. 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 (Initiator). This also means that it is not possible to have more than one PowerBook on a single SCSI

bus in SCSI disk mode.

The second point to be aware of is that the PowerBook computers do not supply termination power. They rely on external

devices to provide termination power on the bus. If there are other devices on the bus that provide termination power, we

recommend that the connection to the first device out of the PowerBook be terminated. It is not required that you do this.

This ensures that the cable is as close to ideal as possible.

Because termination power is not supplied by the PowerBook computers, we also advise that you do not have powered off

devices on the bus. This is true not only for the PowerBook computers but for all Macintosh products. It has been found

that having devices powered off and on the bus causes degradation to performance and signals. A termination problem can

cause incorrect data to be passed during a SCSI transfer. Therefore, the rule is this: Do not have powered off devices

connected to the bus. Also, do not power on a device connected to the bus after booting, and never connect one while the

system is on.

If you still have troubles with the PowerBook after following the termination rules, be sure to check that you have proper

cables and that the other devices on the bus also follow termination rules. Some devices are not following the description

of how Apple's devices work, and following the guidelines of the Apple cable guide does not apply with non-Apple devices. If

you have non-Apple cables, be sure that they meet Apple SCSI specifications. It is possible that other devices on the bus

and improper cabling are what is causing trouble for you.

PowerBook Duo Termination

Unlike the other Macintosh PowerBook computers, the PowerBook Duo models do not rely on an external device to provide

termination power. Both the Deskbar and the DuoDock provide termination power to the bus. They are actively terminated,

which means that they provide termination power and have the best architecture for termination of any other device on the

bus.

Termination Outlined for Each Macintosh

Below is a table that outlines the termination specifics of all Macintosh models (current as of this writing). It also

describes whether termination power (TPWR) is supplied for either the internal or external SCSI bus, depending on

which is present (one or both). The column on the far right describes which terminator is required when the CPU

(currently PowerBook computers only) are in SCSI disk mode and functioning as an external storage device on the

external SCSI chain of a Macintosh. Below the table is an ASCII representation of the active termination concept, which is

new as of the Macintosh IIv, Macintosh Centris, and Macintosh Quadra models.