Seagate ST12400N Product Manual page 60

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11.6.2.1 Physical description
The "N" and "ND" model drives may be daisy-chained together or with other compatible SCSI devices. "N" and
"ND" models use a common cable. Both ends of the cable must be terminated. The model ST12400N and
ST12400NC drives implement single ended drivers and receivers. The model ST12400ND drive implements
differential drivers and receivers. All signals are common between all SCSI devices. The drive may be daisy-
chained only with SCSI devices having the same type drivers and receivers. Devices having single ended inter-
face circuits cannot be on the same daisy-chain with devices having differential interface circuit. A maximum of 8
SCSI devices (including the Host) may be daisy-chained together. The SCSI Devices at both ends of the daisy-
chain are to be terminated. Intermediate SCSI devices shall not be terminated (see Figure 11.6.3-2). Remove the
terminator enble jumper TE, not the terminator power source selector jumper TP (Figure 10.1-1).
"NC" model drives plug into PCB or bulkhead connectors in the Host. They may be connected in a daisy-chain by
the host backplane wiring or PCB circuit runs that have adequate DC current carrying capacity to support the
number of drives plugged into the PCB or bulkhead connectors. A single 80 pin I/O connector cannot support the
DC current needs of several drives, so no daisy-chain cables should be used. A single drive connected via a
cable to a host 80 pin I/O connector is not recommended.
11.6.2.2 Cable requirements ("N" and "ND" models only)
11.6.2.2.1 Single ended I/O circuits ("N" model)
The maximum total cable length for use with model "N" drives (single ended I/O driver and receiver circuits) shall
be 6 metres (19.7 ft.) when operating at line data transfer rates of 5 Mbytes/sec or less, and 3 metres (9.85 ft.)
when operating at transfer rates greater than 5 Mbytes/sec (FAST SCSI). A stub length of no more than 0.1 metre
(0.33 ft.) is allowed off the mainline interconnection with any connected equipment. An ideal impedance match
with cable terminators implies a cable characteristic impedance of 132 ohms (see 11.6.2.2.3). Single ended I/O
pin assignments are shown in Table 11.6.3-1a and 11.6.3-1c.
11.6.2.2.2 Differential I/O circuits ("ND" model)
The maximum total cable length for use with drives having differential I/O drivers and receiver circuits shall be 25
metres (82 ft.). A stub length of no more than 0.2 metre (0.66 ft.) is allowed off the mainline interconnection with
any connected equipment. An ideal impedance match with cable terminators implies a cable characteristic im-
pedance of 122 ohms (see 11.6.2.2.3). Differential I/O pin assignments are shown in Table 11.6.3-1b.
11.6.2.2.3 General cable characteristics
In general, cables having the characteristic impedance's given in 11.6.2.2.1 and 11.6.2.2.2 are not available;
however, impedance's that are somewhat lower are satisfactory. 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 impedance's
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.
Product Manual - Hawk 2 Family SCSI-2 (Volume 1), Rev. C