This article contains frequently asked questions (FAQ) regarding SCSI standards and how they relate to the SCSI cards available from Apple for the Power Macintosh G3 (Blue and White), Power Mac G4 (AGP Graphics) and Power Mac G4 (Gigabit Ethernet), with answers to those questions.

For information on the Power Mac G4 (Gigabit Ethernet) SCSI Card Specifications please see article 58580: Power Mac G4 (Gigabit Ethernet): SCSI Card Specifications .

Questions answered in this article:

1. What is SCSI?
2. What are the differences between the various types of SCSI?
3. Why did Apple remove the SCSI port from the logic board of the new Power Macintosh G3 (Blue and White) and Macintosh Server G3 systems?
4. What SCSI options are available from Apple for the Power Macintosh G3 and G4 systems?
5. What do I do about my older SCSI devices that I want to use?
6. What happens if I connect older SCSI devices to the single-channel SCSI Ultra2 LVD card that ships in my computer?
7. What happens when I mix different types of SCSI devices on the same SCSI channel or bus?
8. Do the Apple SCSI cards support all SCSI devices?

Question 1: What is SCSI?

Answer : SCSI, or Small Computer System Interface, is a standard for connecting peripherals to your computer, including disk and cartridge drives, CD-ROM drives, scanners, and other devices. A SCSI port was included on most Macintosh systems up until the iMac and the 1999 models of the Power Macintosh G3 and Macintosh Server G3.

Question 2 : What are the differences between the various types of SCSI?

Answer : SCSI implementations vary depending on a number of different characteristics. Each type of SCSI can support the transfer of data at a certain speed, measured in megabytes per second. SCSI types are also characterized by the number of devices that can be connected to each channel or bus and the HPbs maximum cable length supported. Different types of SCSI also use different connectors. The chart below summarizes the difference:

http://www.adaptec.com/tools/glossary/general.html

Question 3 : Why did Apple remove the SCSI port from the logic board of the new Power Macintosh G3 (Blue and White) and Macintosh Server G3 systems?

Answer : Apple has provided a new method of connecting peripherals, called FireWire, on every Power Macintosh G3 (Blue and White) and Macintosh Server G3 system. FireWire revolutionizes the way individuals connect high-speed devices to their Macintosh. It provides such features as hot plug capability, support for up to 63 devices, easy cable connectivity, and even bus-supplied power, which make it the perfect successor to SCSI as a means to connect external peripherals. By including FireWire in every Power Macintosh, Apple is providing its customers with high-performance and easy-to-use I/O technology that will be used far into the future.

Question 4 : What SCSI options are available from Apple for the Power Macintosh G3 and G4 systems?

Answer : Apple made a number of SCSI options for optimal hard disk performance and connectivity to older SCSI devices available. Among them:

Ultra SCSI PCI card

The Ultra SCSI PCI card is offered as a build-to-order option at the Apple Store. It works in any 1999 model Power Macintosh G3 or Macintosh Server G3 configuration. The card allows users to connect external SCSI peripherals to the Macintosh, such as scanners, tape drives, and hard disks that use standard SCSI (SCSI-1), Fast SCSI (SCSI-2), and Ultra SCSI (also called Ultra Narrow SCSI). It supports starting up the Macintosh from externally connected devices. The card comes with a high-density 50-pin external connector and a 50-pin to 25-pin SCSI adapter socket to ensure compatibility with existing SCSI cabling. The card also comes with a 50-pin internal connector that is unsupported by Apple. You can connect up to seven external Ultra Narrow devices to the card with a total cable length of 1.5 meters. You can connect up to seven SCSI-1 and SCSI-2 devices to the card with a total cable length of 3 meters. You can also connect a mix of SCSI-1, SCSI-2, and Ultra Narrow external devices, but you are limited to a total cable length of 1.5 meters. See Figure 1.

Figure 1 Ultra SCSI PCI Card Connectors

Single-channel SCSI card (Ultra2 LVD)

The single-channel SCSI card is installed in some configurations of the Power Macintosh G3 and Macintosh Server G3. It is also available through the Apple Store, with a number of hard drive configurations to choose from. This card connects to internal and external Ultra2 LVD SCSI devices. The cable connected to the card provides support for up to three internal Ultra2 LVD SCSI devices. The card supports booting the Macintosh from internal or external devices. It comes with a 68-pin internal connector and a 68-pin external connector. You can connect up to 15 Ultra2 devices to the single-channel SCSI card with a total cable length, both internally and externally, of 12 meters. Apple does not recommend connecting other types of SCSI devices to the single-channel card due to possible cabling limitations or effects on the performance of the internal Ultra2 LVD SCSI hard drives.

Dual-channel SCSI card (Ultra2 LVD) Version 1

The dual-channel SCSI card is offered as a configure-to-order option at the Apple Store with certain hard drive configurations, including the option of three 36GB hard drives, for the Power Macintosh G3 (Blue and White) and Macintosh Server G3. The dual-channel card provides two independent Ultra2 LVD SCSI channels, Channel A and Channel B. Internal Ultra2 LVD SCSI hard drives are connected to the internal Channel B connector. External or internal SCSI devices can be connected to the external or internal Channel A connectors. This card is the perfect all-in-one solution for customers who want to use high-performance internal Ultra2 disk drives and external SCSI peripherals at the same time.

The card comes with two 68-pin internal connectors (Channels A and B), a 50-pin internal connector (Channel B), and a 68-pin external connector (Channel A). You can connect up to 30 Ultra2 SCSI devices to the card with a total cable length, both internally and externally, of twelve meters. You can connect up to 11 Ultra Narrow or Ultra Wide devices with a total cable length, both internally and externally, of 1.5 meters. You can connect up to seven SCSI-1 or SCSI-2 devices to the external channel (Channel A) with a total cable length, both internally and externally, of 3 meters. You can also connect a mix of SCSI-1, SCSI-2, Ultra Narrow, and Ultra Wide devices on the external channel (Channel A), but you are limited to a total cable length, both internally and externally, of 1.5 meters. The internal 50-pin connector (Channel B) is unsupported by Apple. See Figure 2.

Figure 2 Dual Channel Ultra2 LVD (version 1) SCSI Card Connectors

Dual-channel SCSI card (Ultra2 LVD) Version 2
The version 2 dual channel SCSI card is functionally equivalent to the version1 card, though it has been tested and approved for use with the Power Mac G4 (AGP Graphics) computer. Refer to the chart below for more specific technical information. The card is based on the ATTO ExpressPCI UL2D card. You may refer to the ATTO Web site at http://www.attotech.com for more information on it.

Figure 3 Dual Channel Ultra2 LVD (version 2) SCSI Card Connectors

The chart below illustrates many of the technical differences between the various Apple SCSI cards.

Question 5 : What do I do about my older SCSI devices that I want to use?

Answer : Apple offers two SCSI solutions to accommodate the transition between older SCSI devices and FireWire devices. Those options, the Ultra SCSI PCI card and the dual-channel SCSI card, are described in detail above. In addition, a number of vendors are delivering SCSI card solutions for the Power Macintosh G3.

The Adaptec PowerDomain 2930 is an Ultra Narrow SCSI PCI card that supports devices with an external high-density 50-pin connector and an internal standard 50-pin connector (the card ships with an internal SCSI cable). The PowerDomain 2930 supports up to seven internal or external SCSI-1, SCSI-2, and Ultra Narrow SCSI devices. You can boot the new Power Macintosh G3 from internal or external devices with the PowerDomain 2930. For more information, see http://www.adaptec.com/products/overview/pd2930u.html

See Adaptec's Web site for more information on Macintosh SCSI solutions: http://www.adaptec.com/products/solutions/macintosh.html

Question 6 : What happens if I connect older SCSI devices to the single-channel SCSI Ultra2 LVD card that ships in my computer?

Answer : Depending on the type of SCSI device that you connect to the external 68-pin connector on the single-channel SCSI Ultra2 LVD card, you may exceed cable length limitations or slow down the speed of your SCSI bus. For example, connecting an external SCSI-2 drive to the single-channel SCSI card slows the entire Ultra2 SCSI bus from a maximum throughput of 80 MBps to 40 MBps. It also may exceed the maximum cable length of 1.5 meters supported by Ultra Narrow SCSI because the cable connecting the internal Ultra2 hard drive is already nearly 1 meter long. Therefore, you should not connect slower SCSI devices to the single-channel SCSI card.

Question 7 : What happens when I mix different types of SCSI devices on the same SCSI channel or bus?

Answer : Both the speed and cable length limitations of different devices can be affected by mixed SCSI types on a single bus or channel. Any mix of SCSI-1, SCSI-2, Ultra Narrow, and Ultra Wide devices does not affect the maximum throughput of any non-Ultra2 LVD SCSI bus or channel. For example, a SCSI-1 CD-ROM drive, a SCSI-2 Jaz drive, and an Ultra Wide hard drive all connected to an Ultra Wide SCSI bus all operate at their respective speeds (5 MBps for SCSI-1, 10MBps for SCSI-2, and 40 MBps for Ultra Wide) because the Ultra Wide SCSI bus supports up to 40 MBps. Mixing SCSI-1 and SCSI-2 devices with Ultra Narrow and Ultra Wide devices always results in a maximum cable length, both internally and externally, of 1.5 meters.

An Ultra2 LVD SCSI bus or channel with a mix of Ultra2 LVD and non-Ultra2 LVD devices operates at a maximum throughput of 40 MBps, instead of 80 MBps. For example, a SCSI-1 CD-ROM drive, a SCSI-2 Jaz drive, an Ultra-Wide hard drive, and an Ultra2 LVD hard drive all connected to an Ultra2 LVD SCSI bus, operate at their respective speeds except for the Ultra2 LVD hard drive, which will be limited to 40MBps. Mixing Ultra2 LVD devices with non-Ultra2 LVD devices always results in a maximum cable length, both internally and externally, of 1.5 meters.

Question 8 : Do the Apple SCSI cards support all SCSI devices?

Answer : The single-channel and dual-channel SCSI cards do not support all SCSI devices. These cards support only devices that are SCSI Manager 4.3-compliant. The Ultra SCSI PCI card is specifically designed to support older SCSI devices and work with most older devices. To find out whether a specific SCSI peripheral is SCSI Manager 4.3-compliant, contact the vendor of the peripheral for more information.