Chapter 1. Features of the Ultra SCSI Option Board
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## Chapter 1. Features of the Ultra SCSI Option Board

The Ultra SCSI XIO board for the OCTANE workstation provides external SCSI expansion capability. It can be installed in any free slot on the workstation's XIO module that is not occupied by XIO graphics boards. (Graphics boards reside only in slots A or D and A.)

Each of the four controllers on the Ultra SCSI XIO option board supports differential Ultra SCSI, both wide (16 bit) and narrow (8 bit) data transfer rates.

## Other Sources of Information

See the online documentation that shipped with your system for any configuration or programming guides related to the Ultra SCSI board. When looking for a configuration or programming guide, look for a title similar to that of the board protocol name.

Check the release notes for additional information.

## About SCSI Buses and Devices

SCSI devices continue to evolve with higher bus bandwidths, faster data transfer rates, and channels with longer cables and more devices. Specific information about the Ultra SCSI XIO board for the OCTANE workstation is in “About the Ultra SCSI Board for the OCTANE Workstation.” This section provides some general information about SCSI buses and devices.

Table 1-1. Comparison of Ultra and Fast, Wide and Narrow SCSI Rates

| Wide (16 bit) Bus | Narrow (8 bit) Bus
--- | --- | ---
Ultra SCSI (Fast-20) | 40 MB/s (peak theoretical rate) | 20 MB/s (peak theoretical rate)
Fast SCSI | 20 MB/s (peak theoretical rate) | 10 MB/s (peak theoretical rate)

Ultra SCSI is also referred to as Fast-20.

- The data transfer rate is also affected by the media speed of the SCSI peripheral. Fast-20 SCSI is not merely “fast SCSI”. The “-20” in the term signifies the maximum number of megatransfers (millions of 8 bit or 16 bit data transfers per second) that can occur during an I/O operation.

- Data transfer rates are either slow or fast, and are measured in megabytes per second. An operation is either 8 or 16 bit in size; thus, data transfer rates are dependent on bus width.

  | Note: By default the Ultra SCSI board will try to negotiate for wide (16 bit) and for ultra speed.

  Table 1-2. Bus Rate Plus Operating Mode = Data Burst Rate

  Existing Bus | Operating Mode | Data Burst Rate
  --- | --- | ---
  8-bit | Fast | 10 MB second
  8-bit | Ultra | 20 MB second
  16-bit | Fast | 20 MB second
  16-bit | Ultra | 40 MB second

- You may mix Ultra and Fast devices on the same bus. You may also mix 16-bit and 8-bit devices on the same bus. The overall performance of a mixed bus can vary widely.

- To daisy-chain a mix of wide and narrow devices, place the wide devices (as a group) closest to the OCTANE workstation where the bus is wide, and place the narrow devices (as a group) at the end of the daisy chain.

  | Note: Be sure to use an adapter or other special cable that terminates the upper half of the SCSI bus when converting from the final wide device to the first narrow device. Silicon Graphics supplied cables have this integrated into the cable, but not all third-party cables have the terminator integrated into the cable.

- Unsupported devices may not work with the OCTANE workstation, and may cause problems with supported devices.

- The length of combined SCSI cables determines how many devices can be daisy- chained.

- Set addresses manually only on external SCSI devices. See the device manual for setting the address on the device.

- Drives can be easily damaged. Handle your drive carefully; do not drop it or handle it roughly.

## About the Ultra SCSI Board for the OCTANE Workstation

The Ultra SCSI XIO board provides four differential Fast-20 external SCSI channels (0, 1, 2, and 3).

Each bus on the Ultra SCSI board is 16 bits wide. The host controller negotiates with the target SCSI peripheral as to how many bits of data to send in each clock, 8 (narrow SCSI) or 16 (wide SCSI). Each controller also negotiates for Ultra SCSI speeds (synchronous negotiation).

## Differential SCSI Bus Type on the Ultra SCSI Board

A SCSI bus type is either single-ended or differential. All the ports on the Ultra SCSI option board are differential. Peripheral devices are either differential or single ended.

- Differential SCSI peripherals use differential drivers and receivers with built-in hysteresis to provide improved signal noise immunity so that the bus can be driven greater lengths.

- A single-ended SCSI peripheral uses inexpensive, open-collector-wired, OR busing configurations that use a signal comparison to ground. This design limits the distance the signal can be driven because of noise considerations.

Table 1-3 provides information about SCSI types, channels, and cable lengths allowed.

Table 1-3. Maximum Cable Lengths for SCSI Options and Configurations

SCSI Types | Channel | Maximum Cable Length of All Devices Daisy-chained on One Channel
--- | --- | ---
Differential (Fast 10) | 0, 1, 2, 3 | 25 m
Differential (Fast 10, wide) | 0, 1, 2, 3 | 25 m

| Note: Single-ended and differential SCSI peripherals use incompatible integrated circuitry and cannot be mixed on the same bus unless converters are used. No damage will occur, but no devices on the bus will operate.

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Introduction |  | Chapter 2. Installing the Ultra SCSI Option Board