Mac SCSI Overview

Originally published 20 November 2000 as a Mac Daniel column for Low End Mac.

Q: Can you give me a quick overview of Mac SCSI?

A: SCSI, pronounced like "scuzzy," stands for "small computer system interface." It is a parallel data bus, meaning it sends several bits at once. Think of SCSI like an 8-lane highway with a relatively low speed limit. Each SCSI bus will support up to 7 devices, numbered from 0 to 6, with SCSI ID 7 being reserved for the motherboard or add-on SCSI card.

SCSI-1 was the original SCSI specification. It is 8 bits wide, so every time data is transmitted, 8 bits are transmitted at once. SCSI-1 (and SCSI-2) is specified to a 5 MBps (megabytes per second) transfer rate, which is 40 megabits per second, or about four times faster than 10Base-T ethernet. Think of this as 8 trucks driving side-by-side on a highway where the speed limit is 5 miles per hour. SCSI-1 was used only on the Macintosh Plus and had a top speed of 1.5 MBps.

SCSI-2 was a more rigidly defined version of SCSI-1 which made for better reliability and compatibility. At the same time the SCSI-2 spec was defined, Fast SCSI and Wide SCSI were also defined (as well as a combination spec, Fast and Wide SCSI). Fast SCSI raised the speed limit to 10 MPH (10 MBps), while Wide SCSI made the highway 16 lanes wide (16-bits). The combination of the two allowed for a 16-lane highway with a 10 MPH speed limit (20 MBps maximum throughput), which is four times faster than the "narrow" SCSI-2 spec. All Macs from the SE to the Beige G3 have some sort of SCSI-2.

With the exception of the IIfx, the IIci, and the Quadra series, no 68K Mac will generally exceed 1.5 MBps using built-in SCSI. The IIfx has a 3 MBps SCSI-2 bus that supports direct memory access (DMA), helping boost throughput. The IIci will reach speeds of up to 2.1 MBps with its built-in SCSI. Finally, many Quadras can actually reach 6 MBps (though the bus is only specified to 5 MBps) with high-quality cables.

Among the Power Macintosh series, the 7300, 7500, 7600, 8100, 8500, 8600, 9500, and 9600 all have 5 MBps narrow SCSI-2 on their external bus and a second internal bus (with no external connector) that runs Fast SCSI at 10 MBps. These computers will support up to 14 total devices (7 on each bus). Other Power Macintosh computers support the full 5 MBps transfer rate of their onboard SCSI-2 and the standard 7 devices.

All external SCSI connectors on desktop Macs are DB-25 connectors, while on PowerBooks, SCSI is present in the form of a proprietary, squarish 30-pin connector. All internal connectors (where present) on desktop Macs are 50-pin IDCs. Most of the PowerBook line has proprietary internal connectors with combined power and data, and most later PowerBooks use IDE hard disks.

For more information on Mac motherboard SCSI support, check out Apple KBase Archive Article #12315.

SCSI-3 was never used on Mac motherboards and is beyond the scope of this article. Visit Gary Field's SCSI Info Central for more on SCSI-3 variants.

copyright ©2000-2004 by Chris Lawson