SCSI

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SCSI ports have been used in Macintosh computers since the first Macs. SCSI has a wide array of different connections and interfaces. Most common on the Macintosh is the SCSI 25-pin port. SCSI is also used as an internal connection bus for hard drives and CD-ROM drives that most Macintosh systems before 1998 had. Some systems like the Power Macintosh G3 (Beige) have both internal SCSI and IDE connections. Apple stopped including SCSI as a standard connection in favor of IDE, which is cheaper and more popular than SCSI. The SCSI interface can be added to computers made after 1998 via a PCI SCSI interface card. SCSI is faster than IDE, so most PowerMac G3 and early PowerMac G4 systems were able to configured with SCSI cards upon ordering from Apple.

You can add SCSI to a Mac without internal expansion capabilities using a USB or Firewire converter. Currently they are all single-ended, narrow and the fastest ones reach only 20 MB/s because of a lack of better bridge chips and low demand.

There are three signalling systems in parallel SCSI: single-ended (SE), low-voltage differential (LVD) and high-voltage differential (HVD). Some drives are multimode, supporting both SE and LVD operation, but HVD is incompatible with everything else. If you connect both SE and multimode devices on a chain, everything will operate in SE.

The FireWire/SCSI converters on the market only support SE operation.

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