Macintosh Quadra 950: SCSI Support
The internal SCSI channel also supports an active termination which improves SCSI bus signal quality and reliability through improved transmission line and terminator design. This means that only one terminator is needed at the end of the external SCSI chain. The external channel's termination also offers a more uniform impedance and cleaner termination scheme. However, because external cabling and termination are less predictable, performance on the external bus could be lower than the internal. Electrically isolating the two channels ensures that changes in external cabling and termination don't impact the performance of the internal devices.
Even though the internal and external SCSI chains are physically different, they are still logically the same, and can therefore, like all Macintosh systems, accommodate up to seven SCSI devices. You must make sure that every device has a different SCSI ID number, regardless of which SCSI chain the device is connected to. You cannot use SCSI ID 7, and SCSI ID 0 is usually reserved for the internal hard disk. The added value of the Macintosh Quadra 950 dual-controller design is that the internal hard drive(s) performance won't be affected by problematic devices (like a scanner) attached to the external SCSI connector. Improvements in signal quality on the SCSI bus results in higher SCSI reliability.