SCSI cables are to be designed as per the SCSI specifications. Some important points that needs to be considered before using these cables are:
SCSI cables that are for external use should have 25 or 34 twisted pairs of multi-stranded flexible copper wires.
They need to be double shielded and then the cable shield must be connected to the shell of the connectors. However, it should not be connected to any of the ground lines of the SCSI signals.
The nominal differential impedance of the cable used should be in the range of 110 to 135 Ohms.
SCSI signal pairs are assigned to wire pairs as required by the SCSI standard i.e., REQ and ACK in the center, control lines in the middle and data lines at the outside of the cable.
Cables that meet the requirements of higher data transfer rates improve margins for lower data transfer rates.
Some of the problems in the SCSI cable could be:
Conflicting operations.
Less data throughput or even non-operation.
Disks not identified by the SCSI controller.
SCSI controller recognizes the disks but cannot exchange data.
System runs at reduced data throughput.
Occasional bus crashes or error reports created.
Problems can also occur when changing cable length or the peripheral location.
