• The connection at the back of most Apple machines is a DB-25 connector instead of the standard 50-way Centronics.
• Power for the bus termination resistors is not supplied to the connector nor are there any terminators provided for the SCSI circuitry.

When connecting any SCSI peripheral device to an Apple computer, an Apple System Cable will be needed to convert from 25-way to 50-way.

If problems are experienced with the Mac not recognising a drive then there are three options that in all probability will cure the erratic behaviour.

• Disable synchronous data transfer initiation from the drive
• Disable CHECK CONDITION posted on UNIT ATTENTION
• Set the SCSI level of the drive to SCSI-1/CCS

Disabling CHECK CONDITION posted on UNIT ATTENTION will enable a third party disk drive to boot when not connected to the Mac at SCSI ID 0. This ID is normally reserved for the internal drive.

On the subject of third party drives, these are disk drives that were not supplied by Apple therefore Apple's HDSETUP program that comes as part of the Apple System software for formatting and partitioning hard disks will not recognise any such drive. Use third party software to perform this function. The following drivers have been tested and work with Fujitsu drives and all offer different levels of functionality and sophistication.

Notes

Here are a few points to remember when installing disk drives to the Apple External SCSI bus.

There are now three implementations for termination on various Apple Macintosh machines and it should be ascertained before installation of a SCSI device which one is in use.

1. On Macintosh machines where only one internal SCSI device is supported, the internal device and the last device on the external SCSI bus needs to be terminated.

Examples include Macintosh models SE, SE30, IIci, IIcx, IIfx & Quadra 700

2. Some Macintosh machines have two separate SCSI buses. The system software treats them as one virtual bus so unique SCSI IDs are still required for all devices attached. Termination is provided on the main logic board of the computer with , yet again, only the last device on each bus terminated.

Examples include Macintosh models Quadra 900 and 950

3. The last implementation supports multiple internal devices connected on the same external bus. Automatic termination is present on the computer main logic board and so the user need only ensure that both ends of the bus are terminated.

The maximum cable length for the single-ended SCSI bus is 6 metres, although poor quality cable, cables of differing impedances, and the connectors themselves can drastically reduce this figure.

An external disk drive cabinet can account for numerous connectors and perhaps up to half a metre of cable. Don't forget to include in your calculations the SCSI cable inside the Macintosh itself servicing the internal hard disk and CD-ROM where installed.

When using external termination of the SCSI bus, always ensure that at least one device is supplying termination power. Some terminators, especially ACTIVE terminators, have an LED indicating that power is being administered to pin 26 of the interface. Fujitsu disk drives are configured at the factory to supply this power.