Scsi Interface Physical Description; Scsi Interface Cable Requirements; Mating Connectors - Seagate ST446452W Product Manual

Elite 47 Product Manual, Rev. A
9.6.2

SCSI interface physical description

The drives may be daisy-chained together or with other compatible SCSI devices. Both ends of the cable must
be terminated. The drives implement single-ended 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 hav-
ing differential interface circuits. A maximum of 16 SCSI devices (including the host) may be daisy-chained
together. However, please note the restrictions described in Section 9.6.3 about the number of devices allowed
in a daisy chain. The SCSI devices at both ends of the daisy chain are to be terminated. Intermediate SCSI
devices shall not be terminated (see Figure 11). To disable termination, remove the terminator enable jumper
TE on J4A select header, not the terminator power source selector jumper TP on J01 (Figure 7).
9.6.3

SCSI interface cable requirements

In general, cables having the characteristic impedances given in Section 9.6.3.1 are not available; however,
impedances that are somewhat lower are satisfactory. A characteristic impedance as shown in Table 10 is rec-
ommended 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 imped-
ances should not be used in the same bus. Implementations may require trade-offs in shielding effectiveness,
cable length, the number of loads, transfer rates, and cost to achieve satisfactory system operation. If you mix
shielded and unshielded cables within the same SCSI bus, the effect of impedance mismatch must be carefully
considered. Proper impedance matching is especially important to maintain adequate margin at FAST and FAST
-20 (Ultra SCSI) transfer rates.
Elite 47 drives use non-shielded cable connectors. Use a 68-conductor flat cable or 34 twisted pair cable with
connectors listed in 9.6.4.1. Use a minimum conductor size of 28 AWG to minimize noise effects.
Suggested non-shielded flat cable part numbers are:
Flat cable - 35M-3365-68 Twisted pair - Spectra Twist in flat 455-248-68
9.6.3.1 Single-ended I/O circuits
The maximum total cable length allowed depends on several factors. Table 10 lists the maximum lengths
allowed for different configurations of drive usage. These values are from the ANSI SCSI-3 Fast-20 (also called
Ultra SCSI) specification X3T10/1071D. All device I/O lines must have equal to or less than 25 pf capacitance
to ground, measured at the beginning of the stub.
Table 10: Cable characteristics for single-ended circuits
Maximum number of
I/O transfer rate devices on line
<10 M transfers/s 8 (reg. SCSI bus)
<10 M transfers/s 16 (wide SCSI bus)
<20 M transfers/s 4 (reg./wide SCSI bus)
<20 M transfers/s 8 (reg./wide SCSI bus)
Notes:
[1] The spacing of devices on the mainline SCSI bus should be at least three times the stub length (defined
below) to avoid clustering (Refer to Annex C of X3T10/1071D). Based on this criteria, it may be that 8
devices will not actually work on 1.5 meters of line ( .1 x 3 x 7 = 2.1 meters).
A stub length of no more than 0.1 meter (0.33 foot) is allowed off the mainline interconnection with any con-
nected equipment. The stub length is measured from the transceiver to the connection to the mainline SCSI
bus.
Single-ended I/O pin assignments are shown in Table 11.
9.6.4

Mating connectors

Part numbers for connectors that mate with the various Elite 47 I/O connectors are given in the sections follow-
ing.
Maximum cable length
allowed
6 meters (19.7 ft.)
6 meters (19.7 ft.)
3 meters (9.8 ft.)
1.5 meters (4.9 ft.) [1]
Transmission line impedance
REQ/ACK Other signals
90 ± 6 Ohms 90 ± 10 Ohms
90 ± 6 Ohms 90 ± 10 Ohms
90 ± 6 Ohms 90 ± 10 Ohms
90 ± 6 Ohms 90 ± 10 Ohms
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