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Cheetah 18LP Product Manual, Rev. F
9.7.1.1
Single-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 20 lists the maximum lengths allowed for different configurations of drive
usage. These values are from the SPI-2 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 20:
Cable characteristics for single-ended circuits
Maximum number of
I/O transfer rate
devices on the bus
<10M transfers/s
16 (wide SCSI bus)
<20M transfers/s
4 (wide SCSI bus)
<20M transfers/s
8 (wide SCSI bus)
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 and LWV drives are shown in Table 17.
Single-ended I/O pin assignments for LC and LCV models are shown in Table 18. The LC model does not
require an I/O cable. It is designed to connect directly to a back panel connector.
9.7.1.2
Low 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 and LWV model drives are shown in tables 16 and 17. LVD
I/O pin assignments for LC and LCV model drives are shown in tables 18 and 19.
9.7.1.3
General 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 tradeoffs 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.
9.8
Terminator 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 and LWV 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 and LCV drives are designed to be plugged into a backpanel connec-
tor without cabling.
9.9
Terminator power
LW and LWV 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 and LCV drives
These drives cannot furnish terminator power because no conductors in the 80-pin I/O connector are devoted
to terminator power.
Transmission line impedance
maximum cable
REQ/ACK
length allowed
6 meters (19.7 ft)
90 + 6 Ohms
3 meters (9.8 ft)
90 + 6 Ohms
1.5 meters (4.9 ft)
90 + 6 Ohms
Other signals
90 + 10 Ohms
90 + 10 Ohms
90 + 10 Ohms
69
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