The Ultra SCSI external SCSI expander XIO board is a half-height XIO board (marketing code XT-SCSI-4P) that provides expansion capability for the Origin2000 server or Onyx2 workstation. It can be installed in certain XIO slots in those chassis.
Each of the four controllers on the Ultra SCSI option board supports fast and wide SCSI (Fast-20) data transfer rates. The SCSI controller is compliant with ANSI Fast-20 standard X3T10/1071D and ANSI SCSI-2 standard X3.131-1994.
Figure 1-1 shows features of the Ultra SCSI option board.
Figure 1-2 diagrams the Ultra SCSI board.
This chapter describes
Ultra SCSI XIO board outputs
Ultra SCSI XIO board compatible media
cables
SCSI basics
The Ultra SCSI XIO board provides these outputs:
Three differential Fast-20 external SCSI channels (1, 2, and 3)
By default, these channels are terminated on the board; termination can be disabled to allow hot plugging of controllers when a cable with built-in termination is used.
One channel (0) that can be used as one of the following:
external differential Fast-20
internal single-ended Fast-20, which activates internal SCSI for a host in which the Ultra SCSI board is installed in XIO slot 1 instead of an IO6 board
external single-ended Fast-20-capable
If the Ultra SCSI board is not in XIO slot 1, SCSI channel 0 selects the appropriate configuration, depending on the external SCSI device that is connected to it. Termination at the controller is determined automatically by the state of the channel's DIFFSENS signal (pin 16 on the SCSI P connector socket) from the attached external device. If the external device is differential and the differential transceivers are enabled, DIFFSENS is high and the termination at the controller is differential. If the external device is single-ended, DIFFSENS is low and the termination at the controller is single-ended. The channel's differential transceivers are disabled.
When the Ultra SCSI board is in XIO slot 1, its default configuration is external Fast-20.
Fast-20 (“Ultra SCSI”) is a 16-bit wide SCSI bus running at 20 MHz (40 MB/second). Ultra SCSI channel 0 supports
Internal Fast-20 single-ended devices
If the Ultra SCSI board is in XIO slot 1 in an Origin2000 chassis, a socket on the midplane connects the board to the chassis internal SCSI channel. In a system with no IO6 board, this connection enables the internal disk drives and CD-ROM drive to be used.
External devices:
slow, Fast-10, or Fast-20
wide or narrow
single-ended (attached to Ultra SCSI board channel 0 only) or differential
Removable media SCSI devices (typically single-ended) as summarized in Table 1-1
Table 1-1. Supported Removable Media SCSI Devices
Device
Internal
External
CD-ROM drive
x
x
Digital linear tape drive
x
x
8-mm tape drive
x
4-mm digital audio tape drive
x
x
QIC (1/4-inch tape) drive
x
Scanner
x
Color laser printer
x
An external device must be differential, unless it is attached to Ultra SCSI board
channel 0.
Two Y cables included in the option connect the two double ports on the board to as many as four SCSI channels, as diagrammed in Figure 1-3.
Note that the ports and the cable connectors are keyed so that the cables cannot be inserted incorrectly. The high-density double cable connectors that connect to the ports are labeled A and B on the wide-keyed side:
A corresponds to channel 0 (top connector) and 2 (bottom connector)
B corresponds to channel 1 (top connector) and 3 (bottom connector)
The labels on the cables near the P (standard 68-pin SCSI) connectors indicate the XIO slot, and SCSI channel. In a multi-module system, a label designating the module (host) can also be present.
This section explains
SCSI channels and devices
SCSI buses
configuring a SCSI channel
The Ultra SCSI board supports four channels; each channel can support an Origin Vault storage option. Each Origin Vault can contain up to six 4.5 GB or 9.1 GB 3.5-inch disk drives as well as one or two 5.25-inch SCSI-2 peripherals.
SCSI devices continue to evolve with higher bus bandwidths, faster data transfer rates, and channels with longer cables and more devices. A protocol establishes a SCSI bus's bandwidth, type, and data transfer rate. These factors are interdependent. For example, a 16-bit SCSI peripheral typically transfers more data at a faster rate than an 8-bit SCSI peripheral.
This section explains
SCSI bus bandwidth
SCSI IDs
SCSI bus type
data transfer rates
A bus on an Ultra SCSI board is 16 bits wide. The host system 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). Regardless of how the bus is utilized, it remains 16 bits wide.
Since the default SCSI bus controller is 0, 8-bit buses use seven SCSI IDs for devices, and 16-bit buses use 15 SCSI IDs. For an Ultra SCSI board attached to two Origin Vaults, the first Origin Vault's 3.5-inch disks are numbered 1 through 6 and the second Origin Vault's disks are numbered 9 through 14.
In narrow mode, there may be eight target SCSI IDs, minus the number of IDs for hosts on the bus. The target IDs must not conflict with the host ID; if there is more than one host, their IDs also must not conflict with each other. Similarly, in wide mode, there are 16 target SCSI IDs minus the number of hosts. (The default SCSI bus controller ID can be reconfigured for dual-hosted SCSI operation.)
A SCSI bus type is either single-ended or differential. 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.
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. Table 1-2 summarizes maximum cable lengths for the various SCSI options.
Table 1-2. Maximum Cable Lengths for SCSI Options
SCSI Flavor | Channel | Maximum Cable Length |
|---|---|---|
Differential Fast-20 | 0, 1, 2, 3 | 25 m |
Differential Fast-10 wide | 0, 1, 2, 3 | 25 m |
Single-ended | 0 | 6 m |
Single-ended Fast-10 | 0 | 3 m |
Single-ended Fast-20 | 0 | 1.5 m |
Single-ended Fast-20 can support a maximum of eight devices, which must be evenly spaced along a 1.5-meter cable; a 3-meter cable can support a maximum of only four devices, evenly spaced. For Ultra SCSI boards in the Origin family of systems, single-ended Fast-20 is used exclusively inside the host system; differential Fast-20 is used for external SCSI buses.
 | Caution: Single-ended and differential SCSI peripherals use incompatible integrated circuitry and cannot be mixed on the same bus, unless converters are used. |
Data transfer rates are either slow or fast, measured in megabytes per second. An operation is either 8- or 16-bit in size; thus, data transfer rates are dependent on bus bandwidth. Figure 1-4 compares data transfer rates.
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 (number of million operations per bus cycle, based on a bus's burst data rate) that can occur during an I/O operation, doubling the data rate. For example, an existing narrow 8-bit SCSI bus operating in fast mode can burst data at rates of 10 MB per second; the same bus running Fast-20 SCSI can burst data at 20 MB per second. An existing wide 16-bit SCSI bus in fast mode can burst data at 20 MB/sec, but the corresponding bus operating Fast-20 SCSI can burst data at 40 MB/sec.
These guidelines reflect the dependencies among SCSI bus bandwidths, types, and data transfer rates. Follow these guidelines help to minimize inconsistent or inoperable SCSI buses:
Install single-ended and differential devices on separate buses.
In calculating SCSI bus length, include all cable length inside devices, components, and chassis.
For maximum performance, install wide and narrow SCSI devices on different buses.
To configure a SCSI channel, you must verify the type of SCSI protocol required (single-ended/differential, data transfer rate), identify each component in the SCSI channel, and obtain any missing components.
Because the system controller negotiates independently with the devices on a bus to establish the acceptable transfer rate of each device, you can mix fast and slow devices on the same bus. However, mixing narrow Fast-20 and wide Fast-20 devices on the same single-ended SCSI bus is not recommended.