60 | Barracuda 36ES2 Product Manual, Rev. B |
9.6.2SCSI interface physical description
The drive models described by this product manual support the physical interface requirements of the Ultra160 SCSI Parallel Interface-3 (SPI-3) standards as defined in American National Standard document X3T10/1302D revision 14, and operate compatibly at the interface with devices that support earlier SCSI-2 and SCSI-3 stan- dards. It should be noted that this is only true if the systems engineering has been correctly done, and if earlier SCSI-2 and SCSI-3 devices respond in an acceptable manner (per applicable SCSI Standards) to reject newer Ultra160 SCSI protocol extensions that they don’t support.
The drives documented in this manual support single-ended and low voltage differential physical interconnects (hereafter referred to as SE and LVD, respectively) as described in the ANSI SPI-3 standard. These drives implement driver and receiver circuits that can operate either SE or LVD. However, they cannot switch dynam- ically between SE and LVD operation.
The drives typically operate on a daisy-chain interface in which other SCSI devices are also operating. Devices on the daisy chain must all operate in the same mode, either SE or LVD, but not a mixture of these. On the interface daisy chain, all signals are common between all devices on the chain, or bus, as it is also called. This daisy chain of SCSI devices must be terminated at both ends with the proper impedance in order to operate correctly. Do not terminate intermediate SCSI devices. In some cases, the SCSI devices at each end have onboard termination circuits that can be enabled by installation of a jumper plug (TE) on the device. These ter- mination circuits receive power from either a source internal to the device, or from a line in the interface cable specifically powered for that purpose. LW model drives do not have onboard termination circuits. Some type of external termination circuits must be provided for these drives by the end user or designers of the equipment into which the drives will be integrated. See Standard X3T10/1302D, sections 6.4 and 6.5 for the maximum number of devices that can successfully operate at various interface transfer rates on SE and LVD daisy chains.
Table 12 shows the interface transfer rates supported by the various drive models defined in this manual.
Table 12: Interface transfer rates supported
Interface type/ | Maximum transfer rate | | | | |
| | | | | |
drive models | Asynchronous | Fast-5 | Fast-10 | Fast-20 | Fast-40 | Fast-80 |
| | | | | | | |
| | | | | | | |
SE | ST336918N | yes | yes | yes | yes | no | no |
| | | | | | | |
LVD | ST336938LW | yes | yes | yes | yes | yes | yes |
| | | | | | | |
9.6.3SCSI interface cable requirements
The characteristics of cables used to connect Ultra160 SCSI parallel interface devices are discussed in detail in Section 6 of ANSI Standard X3T10/1302D. The cable characteristics that must be considered when inter- connecting the drives described in this manual in a Ultra160 SCSI parallel, daisy-chain interconnected system are:
•characteristic impedance (see Section 6.1)
•propagation delay (see Section 6.1)
•cumulative length (see sections 6.4 and 6.5)
•stub length (see sections 6.4 and 6.5)
•device spacing (see sections 6.4 and 6.5)
To minimize discontinuances 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, number of loads and spacing, 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-20 and FAST-40 SCSI transfer rates.
Note. For LVD operation, twisted pair cables are recommended. For LVD Fast-40 operation, twisted pair cables are strongly recommended. For Fast-80 operation, twisted pair cables are required.