• SCSI Interface Physical Description
• SCSI Interface Cable Specifications
• SCSI Interface Cable Requirements
• Single-Ended I/O circuits ("N" and "W" models)
• Differential I/O Circuits ("WD" and "DC" models)
• LVD - Low Voltage Differential ("LC" & "LW" models)

SCSI Interface Physical Description

Daisy-Chaining

• The drives may be daisy-chained together or with other compatible SCSI devices.
• The drive may be daisy-chained only with SCSI devices having the same type drivers and receivers.
• Devices having single-ended interface circuits cannot be on the same daisy-chain with devices having differential interface circuits ("DC", "ND", and "WD" models).
• A maximum of 8 ("N" model) or 16 ("W" and "WC" model) SCSI devices (including the host) may be daisy-chained together. However, please note the restrictions described in "Single-Ended I/O Circuits" about the number of devices allowed in a daisy-chain.
• The SCSI devices at both ends of the daisy-chain are to be terminated.

Termination

• Both ends of the cable must be terminated.
• The SCSI devices at both ends of the daisy-chain are to be terminated.
• Termination Power (TP) must be set to meet the requirements, if any, of the host adapter or backplane.
• Intermediate SCSI devices shall not be terminated. Remove the "terminator enable" jumper (TE) (or set the "terminator disable" jumper (TD) on newer models), or remove the external terminators (older models), not the terminator power source selector jumper (TP).

Signals, Drivers and Receivers

• The "N," "W," and "WC" model drives implement single-ended drivers and receivers.
• All signals are common between all SCSI devices.

Connecting a "WC" or "DC" Model Drive

"WC" and "DC" model drives plug into PCB or bulkhead connectors in the host. They may be connected in a daisy-chain by the host backplane wiring or PCB circuit runs that have adequate DC current carrying capacity to support the number of drives plugged into the PCB or bulkhead connectors. A single 80-pin I/O connector cable cannot support the DC current needs of several drives, so no daisy-chain cables beyond the bulkhead connectors should be used. Termination is provided by the backplane (bulkhead), so there is no termination available on the drives. A single drive connected with a cable to a host 80-pin I/O connector is not recommended. For WC drives, an acceptable alternative is to utilize an 80-pin SCA to 68-pin Wide adapter to connect a WC drive to a 68-pin Wide SCSI bus.

SCSI Interface Cable Specifications

The ANSI Standard X3T10/1142D (section 6) discusses the cable characteristics that must be considered when interconnecting the drives described in a SCSI-3 parallel, daisy-chain interconnected system, which are:

• characteristic impedance (see section 6.1)
• propagation delay (see section 6.1)
• cumulative length (see sections 6.4 and 6.5)
• stub length (see sections 6.4 and 6.5)
• device spacing (see sections 6.4 and 6.5)

For specifications and requirements not discussed in this document, refer to the above mentioned ANSI Standard.

SCSI Interface Cable Requirements

In general, cables having the characteristic impedances given in the section titled "Single-ended I/O circuits ("N" and "W" models)" below are usually not available; however, impedances that are somewhat lower are satisfactory. A characteristic impedance as listed in the table below is recommended for an unshielded flat cable or twisted pair ribbon cable.

To minimize discountenances and signal reflections, cables of different impedances should not be used in the same bus. Implementations may require trade-offs in shielding effectiveness, cable length, number of loads and spacing, transfer rates, and cost to achieve satisfactory system operation. If shielded and unshielded cables are mixed within the same SCSI bus, the effect of impedance mismatch must be carefully considered. Proper impedance matching is especially important in order to maintain adequate margin at Ultra SCSI (FAST-20) and Ultra2 SCSI (FAST-40) transfer rates.

Single-Ended I/O Circuits ("N" and "W" models)

The maximum total cable length allowed with drives having single-ended I/O driver and receiver circuits depends on several factors. The table below lists the maximum lengths allowed for different configurations of drive usage.

Note: While the Ultra-SCSI and SCSI-3 Ultra drives (narrow and wide) will transfer data at rates of < 20 MegaTransfers/sec (A "MegaTransfer" is a unit of measure referring to the rate of signals on the interface regardless of the width of the bus.), and the interfaces will support up to 8 narrow devices or 16 wide devices, the ANSI SCSI-3 Fast-20 (Ultra SCSI) specification X3T10/1071D will not support the full number of supported devices on the cabling. To successfully attach more than 4 devices, a backplane-based configuration is required. At this time, Ultra SCSI data transfer rates limit the bus to 8 devices.

A stub length of no more than 0.1 meter (0.33 foot) is allowed off the mainline interconnection with any connected equipment. The stub length is measured from the transceiver to the connection to the mainline SCSI bus.

Differential I/O Circuits ("WD" and "DC" models)

The maximum total cable length allowed with drives having differential I/O driver and receiver circuits is 25 meters (82 feet). Twisted-pair cable (either twisted-flat or discreet wire twisted pairs) should be used with differential I/O transceiver circuits. A stub length of no more than 0.2 meters (0.66 feet) is allowed off the mainline interconnection with any connected equipment. The stub length is measured from the transceiver to the connection to the mainline SCSI bus. The spacing of devices on the mainline SCSI bus should be at least three times the stub length to avoid clustering (Refer to Annex C of X3T10/1071D). This restriction easily allows the 16 device IDs that the SCSI 16 bit wide bus can address. These values are from the ANSI SCSI-3 Fast-20 (Ultra SCSI) specification X3T10/1071D.

LVD - Low Voltage Differential ("LC" & "LW" models)

Twisted-pair cable (either twisted-flat or discreet wire twisted pairs) should be used with low voltage differential I/O transceiver circuits. A stub length of no more than 0.2 meters (0.66 feet) is allowed off the mainline interconnection with any connected equipment. The stub length is measured from the transceiver to the connection to the mainline SCSI bus. The spacing of devices on the mainline SCSI bus should be at least three times the stub length to avoid clustering (Refer to Annex C of X3T10/1071D). This restriction easily allows the 16 device IDs that the SCSI 16 bit wide bus can address. These values are from the ANSI SCSI-3 Fast-20 (Ultra SCSI) specification X3T10/1071D. The LVD interface will support the Ultra2 (Fast-40) with its external transfer rate of 80 Mbytes per second on wide transfers and can handle up to 12 meters of bus length on a multidrop (more than 2 devices) bus or 25 meters on a point-to-point (2 devices) hookup.