This article tells how to add peripheral devices to your PowerBook and how to use SCSI Disk Mode, which lets some PowerBook computers connect to a desktop Macintosh and appear as an external hard disk.

The SCSI Port and Cabling

The SCSI port used in PowerBook computers is called an HDI-30. It is a square connector with 30 pins. There are two different SCSI cables. In addition to one of these cables, you may need a terminator, model number M3503xx/A (in the US all model numbers are LL, thus the terminator would be M3503LL/A). Refer to article 24281: " PowerBook: SCSI Cables Used " for images of the various cables.

Apple HDI-30 SCSI Disk Adapter
This adapter, model number M2539xx/A is specifically for SCSI Disk Mode, also known as HD Target Mode. It can be distinguished by its HDI-30 connector having 30 pins (5 rows of 6). The connector on the other end is a standard Centronics 50 connector.

Apple HDI-30 SCSI System Cable
This cable is for connecting external SCSI devices, like CD-ROMs and hard drives, to your PowerBook and it is called the Apple HDI-30 SCSI System Cable, model M2538xx/A. Its HDI-30 connector only has 29 pins. The connector on the other end is a standard Centronics connector.

SCSI Disk Mode (HD Target Mode)

This mode enables the PowerBook hard drive appear on the Finder desktop of the host Macintosh it is connected to. This mode is supported on all PowerBook models with the exception of the 140, 145, 145B, 150, and 170 and is recommended for high speed transfer of information, but is not necessarily intended for long-term usage.

On PowerBook computers equipped with IDE hard drives, such as the PowerBook G3 Series, this functionality is called HD Target Mode though functionally it is the same. Follow these steps to set up SCSI disk mode:

1. In the PowerBook control panel (Portable control panel for the 100) choose the SCSI ID for the PowerBook. Remember, each SCSI device must have a unique ID number. Verify that the hard drive name for the PowerBook is different from the hard drive name of the host computer.

2. Shut down the PowerBook and any additional SCSI devices in the SCSI chain. Attach the HDI-30 SCSI Disk Adapter cable to the PowerBook and a terminator at the other end (the terminator may not be required; check your user manual).

3. Plug the SCSI Disk Adapter cable to either a SCSI cable attached to the desktop Macintosh, or to any existing SCSI device on the SCSI chain.

4. Power on the PowerBook. You should see a diamond shaped symbol moving across the screen with a number corresponding to the SCSI ID you set. Once you see that, power on the desktop computer. After that computer starts up, you should see the PowerBook computer's hard drive appear on the Finder desktop of the host Macintosh.

5. If you find that the host computer does not start up properly, turn both computers off, remove the terminator and then restart both computers.

There are many devices that use the SCSI port to connect to your PowerBook. Some examples are hard disks, CD-ROM drives, scanners, and backup devices.

The most important thing to keep in mind when adding devices to a SCSI chain is the ID number. The SCSI chain has an ID range from 0 to 7. ID 7 is reserved for the PowerBook itself; ID 0 is reserved for the internal hard drive on most computers. The other 6 ID numbers are for external devices. Generally, this ID can be set on the device by some external selector -- consult the manual for the device to see how to set this number. The ID number for each device must be unique on the chain. That is, only one device can use a given number. If you are running the Apple System Profiler, the Device Information section will give you more information about which ID numbers are in use.

Officially a SCSI device chain must be terminated at both the beginning and end. Some older PowerBook computers do not supply termination at the begining of the SCSI chain. Because of this, a second terminator may be needed before the first device in the chain. Check your PowerBook's user manual to determine the placement and quantity of SCSI terminators needed for your PowerBook model in the section titled, "Connecting SCSI devices."

If there is more than one external SCSI device connected in a chain to a PowerBook computer, they can be connected together using the Apple SCSI Peripheral Interface Cable. This cable is a Centronics 50 pin to 50 pin cable. Both the first and last device in the chain need one terminator each, unless one or more of the devices in the chain have termination built in.

Some older SCSI devices are internally terminated. If you are not sure, it is very important that you check the manual for each device. If any of the devices in the middle are terminated, your system will have difficulties seeing all the devices.

Before connecting or disconnecting devices on a SCSI chain, turn the power off for all devices. When you get ready to power up, turn on all external devices first, then turn on the PowerBook. Unpowered device may not supply power to their terminator. When you wish to end the session, shut down the PowerBook first, then turn off all the devices.

Common Problems Q & A

Question : I am attempting SCSI Disk Mode, and the PowerBook does not display the diamond pattern. What is wrong?

Answer : Some PowerBook computers will not show the disk mode icon until the desktop Macintosh is powered on.

Make sure you are using the correct cable. This is the most common difficulty. Refer to the previous Cabling section to verify that you are using the SCSI Disk Adapter cable. Second, be aware that SCSI Disk Mode is only supported in certain PowerBooks. Review the instructions under SCSI Disk Mode to see which PowerBooks are supported.

Be sure to use Apple cables because some third party cables may not have adequate shielding.

Question : I have added an external SCSI device to my PowerBook and when I start up, I get a diamond symbol on my screen. What does this mean?

Answer : You are using the SCSI Disk Adapter Cable instead of the System Cable. See the preceding section on Cabling to see the difference.

Question : I have added an external SCSI device to my PowerBook and when I start up, I get only a flashing question mark (?) over a disk icon. Why?

Answer : Something is preventing your PowerBook from starting up correctly. First, try to start up your computer from its Disk Tools disk or Mac OS system software CD. If it starts up and your hard drive does not appear on the desktop, you may have a SCSI ID conflict. Make sure that the external device is set to an ID number from 1 to 6, and if more than one device is connected, make sure each has a unique ID number. Try opening the Apple HD SC Setup program on the Disk Tools disk or the Drive Setup program on the Mac OS system software CD. If that program can see your hard drive, try selecting the Update button.

This updates the driver software on the hard disk. Check the version of the Drive Setup program. It is possible that a newer version is available which may address your situation. Make sure you are configuring your SCSI chain correctly; refer to the previous section, paying close attention to termination.

Question : In SCSI Disk Mode, the desktop computer tries to start up from the PowerBook computer's drive, rather than its own internal drive. An example is an error message such as the one saying the system software is not correct for this Macintosh. What should I do?

Answer : Power the desktop computer off, and then start up from the Disk Tools disk for that computer. Make sure that both the PowerBook drive and the desktop's internal drive both show up on the screen. Verify that the names of the two drives are different. Open the Startup Disk control panel and verify the desktop computer is selected. Restart and let the Disk Tools disk be ejected; it should then start up from the correct drive.