Seagate ST19171DC, Product Manual

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56 Barracuda 9 Product Manual, Rev. C
9.6.3.1 Single-ended I/O circuits (“N,” “W,” and “WC” models)
The maximum total cable length allowed with drives having single-ended I/O driver and receiver circuits
depends on several factors. Table 12 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.
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 ( .1m x 3 x 7 = 2.1 meters; 8 devices would be con-
nected by 7 cable sections).
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 Tables 13, 14, and 16.
9.6.3.2 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 differ-
ential I/O transceiver circuits. A stub length of no more than 0.2 meter (0.66 foot) is allowed off the mainline
interconnection with any connected equipment. The stub length is measured from the transceiver to the con-
nection 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
(also called Ultra SCSI) specification X3T10/1071D.
An ideal impedance match with cable terminators implies a cable characteristic impedance of 122 ohms. Dif-
ferential I/O pin assignments are shown in Tables 15 and 17.
Table 12: Cable characteristics for single-ended circuits
I/O transfer rate
Maximum number of
devices on line Maximum cable
length allowed
Transmission line impedance
REQ/ACK Other signals
<10 M transfers/s 8 (reg. SCSI bus) 6 meters (19.7 ft.) 90 ± 6 Ohms 90 ± 10 Ohms
<10 M transfers/s 16 (wide SCSI bus) 3 meters (9.8 ft.) 90 ± 6 Ohms 90 ± 10 Ohms
20 M transfers/s 4 (reg./wide SCSI bus) 3 meters (9.8 ft.) 90 ± 6 Ohms 90 ± 10 Ohms
20 M transfers/s 8 (reg./wide SCSI bus) [1] 1.5 meters (4.9 ft.) 90 ± 6 Ohms 90 ± 10 Ohms