Seagate ST318405LW/LC manual Low voltage differential I/O circuits, General cable characteristics

Cheetah 36XL Product Manual, Rev. E 71

9.7.1.1Single-ended drivers/receivers

The maximum total cable length allowed with drives using single-ended I/O driver and receiver circuits depends on several factors. Table 17 lists the maximum lengths allowed for different configurations of drive usage. These values are from the SPI-3 document. 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 17: 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 14.

Single-ended I/O pin assignments for the LC models are shown in Table 15. The LC models do not require an I/O cable. They are designed to connect directly to a back panel connector.

9.7.1.2Low 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 13 and 14. LVD I/O pin assignments for LC model drives are shown in tables 15 and 16.

9.7.1.3General 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.

9.8Terminator requirements

Caution: These drives do not have onboard internal terminators. The user, systems integrator or host equip- ment manufacturer must provide a terminator arrangement external to the drive when termination is required. For LW drives, terminator modules can be purchased that plug between the SCSI I/O cable and the drive I/O connector or on the end of a short I/O cable stub extending past the last cable connector. LC drives are designed to be plugged into a backpanel connector without cabling.

9.9Terminator power

LW drives

You can configure terminator power from the drive to the SCSI bus or have the host adaptor or other device supply terminator power to the external terminator. See Section 8.1 for illustrations that show how to place jumpers for this configuration.

LC drives

These drives cannot furnish terminator power because no conductors in the 80-pin I/O connector are devoted to terminator power.