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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 "N" and "W" model 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 having differential interface circuits. A maximum of 8 ("N" models) or 16 ("W" or "WD" models)
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
14). Remove the terminator enable jumper TE on J4A select header ("N" and "W" models), or the external ter-
minators ("WD" models), not the terminator power source selector jumper TP on J01 (Figures 8 and 9a).
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.
"N" models use non-shielded cable connectors. Use a 50-conductor flat cable or 25 twisted pair cable. Use a
minimum conductor size of 28 AWG to minimize noise effects.
Suggested non-shielded flat cable part numbers are:
Flat cable - 35M-3365-50 Twisted pair - Spectra Twist in flat 455-248-50
"W" and "WD" models use non-shielded cable connectors. Use a 68-conductor flat cable or 34 twisted pair
cable with connectors listed in 9.6.4.2. 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 ("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. Table 11 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 11:
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).
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]
Elite 23 Product Manual, Rev. A
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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