Seagate HAWK 4, Product Manual

Page: 52 / 68

44 Product Manual - Hawk 4 Family SCSI-2 (Volume 1), Rev. F
______________________________________________________________________________________
11.6.2.1 Physical description
The drives may be daisy-chained together or with other compatible SCSI devices using a common cable.
Both ends of the cable must be terminated. The model "N" drive implements single ended drivers and receiv-
ers. The model "ND" drive implements differential drivers and receivers. All signals are common between all
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 circuit. A maximum of 8 SCSI devices (including the Host) may be daisy-chained
together. The SCSI Devices at both ends of the daisy-chain are to be terminated. Intermediate SCSI devices
shall not be terminated (see Figure 11.6.3-2). Remove the terminator enable jumper TE, not the terminator
power source selector jumper TP (Figure 10.1-1).
11.6.2.2 Cable requirements
11.6.2.2.1 Single ended I/O circuits ("N" models)
The maximum total cable length for use with drives having single ended I/O driver and receiver circuits shall
be 6 metres (19.7 ft.) when operating at line data transfer rates of 5 Mbytes/sec or less, and 3 metres (9.85 ft.)
when operating at transfer rates greater than 5 Mbytes/sec (FAST SCSI). A stub length of no more than 0.1
metre (0.33 ft.) is allowed off the mainline interconnection with any connected equipment. An ideal imped-
ance match with cable terminators implies a cable characteristic impedance of 132 ohms (see 11.6.2.2.3).
Single ended I/O cable pin assignments are shown in Table 11.6.3-1a.
11.6.2.2.2 Differential I/O circuits ("ND" models)
The maximum total cable length for use with drives having differential I/O drivers and receiver circuits shall be
25 metres (82 ft.). A stub length of no more than 0.2 metre (0.66 ft.) is allowed off the mainline interconnection
with any connected equipment. An ideal impedance match with cable terminators implies a cable character-
istic impedance of 122 ohms (see 11.6.2.2.3). Differential I/O pin assignments are shown in Table 11.6.3-1b.
11.6.2.2.3 General cable characteristics
In general, cables having the characteristic impedance’s given in 11.6.2.2.1 and 11.6.2.2.2 are not available;
however, impedance’s that are somewhat lower are satisfactory. A characteristic impedance of 100 ohm
±10% is recommended for unshielded flat or twisted pair ribbon cable. However, most available cables have
a somewhat lower characteristic impedance. To minimize discontinuances and signal reflections, cables of
different impedance’s should not be used in the same bus. Implementations may require trade-offs in shield-
ing effectiveness, cable length, the number of loads, 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 main-
tain adequate margin at FAST SCSI transfer rates.