Seagate ST318438LW manual Low voltage differential I/O circuits, General cable characteristics

Barracuda 36ES2 Product Manual, Rev. B				71
Table 16: Cable characteristics for single-ended circuits
	Maximum number of	Maximum cable	Transmission line impedance
I/O transfer rate	devices on the bus	length allowed	REQ/ACK	Other signals
<10M transfers/s	16 (wide SCSI bus)	6 meters (19.7 ft)	90 + 6 Ohms	90 + 10 Ohms
<20M transfers/s	4 (wide SCSI bus)	3 meters (9.8 ft)	90 + 6 Ohms	90 + 10 Ohms
<20M transfers/s	8 (wide SCSI bus)	1.5 meters (4.9 ft)	90 + 6 Ohms	90 + 10 Ohms

A stub length of no more than 0.1 meter (0.33 ft) is allowed off the mainline interconnection with any connected equipment. The stub length is measured from the transceiver to the connection to the mainline SCSI bus.

Single-ended I/O cable pin assignments for LW drives are shown in Table 15.

9.7.1.3Low voltage differential I/O circuits

The maximum total cable length for use with drives using LVD I/O drivers and receiver circuits is 12 meters (39.37 ft.). A stub length of no more than 0.1 meter is allowed off the mainline interconnection with any con- nected equipment. LVD I/O pin assignments for LW model drives are shown in tables 14 and 15.

9.7.1.4General cable characteristics

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 discontinuities and signal reflections, cables of different impedances 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 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 maintain adequate margin at fast SCSI transfer rates.