SCSI Manager

As Apple adopted SCSI before the ANSI standard was ratified, Apple’s implementation deviates from the ANSI standard. Because of this, devices have to be configured a little differently for them to work with a Macintosh. Differences include:

  • Though parity is generated by the host Macintosh for compatibility, it does not check if parity is valid nor issue retries if parity is invalid
  • Unit Attention causes startup problems with some older Macs (primarily the Mac Plus) and should be disabled to prevent issues
  • Synchronous negotiation is not supported
  • Auto-spin should be enabled for convenience
  • Spindle synchronization should be disabled
  • SCSI-2 devices must be set to SCSI-1 command compatibility mode
  • Packet-based devices (such as CD burners) won’t work reliability

    SCSI Manager 4.3

    SCSI Manager 4.3 was introduced with the Macintosh Quadra 840AV and is far more compliant with the ANSI SCSI standard. However, many entry-level Macintoshes did not support it as Apple used older SCSI controllers in those models. An add-in expansion card, such as the FWB SCSI JackHammer, would enable support for it. SCSI Manager 4.3 supports:

  • The SCSI-2 command set
  • Parity
  • DMA I/O
  • LUNs
  • Multiple physical SCSI buses
  • Logical device disconnect/reconnect
  • Synchronous negotiation
  • Unit Attention
  • Packet-based devices such as CD burners

    Termination

    Termination is tricky. It will take a few attempts to create a working configuration.

    Notes on Terminators:

  • SCSI is an electrical bus and has to be terminated at each end
  • A terminator pack can be plugged into the daisy chain port of an external device, between the cable and the external device, or to the last plug on an internal SCSI cable
  • Older hard drives have a row of removable resistor packs for termination, newer drives have integrated resistors with a jumper setting to enable or disable termination
  • To prevent confusion, a hard drive’s built-in terminator should be disabled if it is placed in an external enclosure
  • Enable termination on an internal hard drive if it is the only internal SCSI device or if it is the last device on the cable; disable termination if it is not the last device on the cable
  • If no device is connected to the external SCSI port, the port is automatically terminated; internal ports are also automatically terminated if no device is attached
  • Some high-end Macintoshes have separate internal (Fast) and external (Slow) SCSI busses; they must be terminated separately as they are not electrically connected

    Notes on Termination Power:

  • Termination power is +5V DC that is applied to pin 26 of the SCSI cable and powers the termination resistors on the bus
  • Termination power must be supplied by at least one of the SCSI devices on the bus
  • The SCSI controller may or may not provide termination power; most PCI SCSI controller cards do.
  • Macintoshes that originally shipped with an internal SCSI hard drive provide termination power to the bus of the built-in controller
  • PowerBooks’ SCSI controllers do not provide termination power. Instead, the PowerBook’s internal hard drive must provide termination power to the bus. External devices, when connected, must provide their own termination power.
  • Macintoshes that shipped with an internal ATA hard drive do not provide termination power. Exception: PCI-based PowerBooks (3400, 2400, and G3) and PowerBook Duos do provide termination power on the external bus (since the docks of the Duos contain the external bus SCSI controller, they also provide termination power)
  • The Macintosh Plus and Macintosh Portable do not provide termination power to any device

    Specific tips on configuring SCSI devices

    PowerBooks

  • Use two terminators when connecting a SCSI device to a PowerBook, one as a passthrough on the cable between the PowerBook and the first device and another at the end of the chain
  • Enable termination power on an external device when connecting it to a PowerBook, as the PowerBook does not provide termination power
  • PowerBooks in Target Disk Mode must be at the end of the SCSI chain

    Macintosh IIfx

  • The Macintosh IIfx requires a special black Apple SCSI Cable Terminator II for external devices
  • Only one Terminator II should be connected at any one time