Seagate ST3285N, Product Manual

Page: 29 / 151

2.5 Cable requirements
The characteristic impedance of the cable should be between 90 ohms
and 140 ohms. However, most available cables have a somewhat lower
characteristic impedance. To minimize discontinuities and signal reflec-
tions, do not use cables of different impedances in the bus.
Your design may require trade-offs in shielding effectiveness, the length
of the interface cable, the number of loads, and the transfer rates. If your
design uses both shielded and nonshielded cables within the same SCSI
bus, you must allow for the effects of impedance mismatch.
To minimize noise effects, use a minimum conductor size of 28 AWG.
Use only nonshielded cable connectors. Use a 50-conductor flat cable
or 25-conductor twisted-pair cable. The recommended nonshielded flat
cable part numbers are shown in the following table:
Part Manufacturer
Flat Cable 3M-3365-50
Twisted Pair Spectra Twist-N-Flat 455-248-50
2.6 Single-ended cable
The single-ended SCSI cable must meet the following requirements:
•
The cable cannot be longer than 6.0 meters.
•
A cable stub cannot be longer than 0.1 meter, from the mainline
interconnection to any device. Stubs must be separated by at least
0.3 meter.
2.6.1 Fast synchronous data transfer
When using fast synchronous data transfer rates, the SCSI interface
cable must meet the following additional requirements:
•
The cable cannot be longer than 3.0 meters.
•
A characteristic impedance of 90 ohms to 132 ohms is recommended
for nonshielded flat cable or twisted-pair ribbon cable.
•
The signal attenuation at 5 MHz must not be greater than 0.095 dB
per meter.
•
The DC resistance at 20°C must not exceed 0.230 ohms per meter.
•
The propagation delay delta of a shielded, twisted-pair cable must not
exceed 20 nsec per meter.
ST3655 Family SCSI Drives Product Manual, Rev. A 19