The Basics of SCSI Cabling

The SCSI specification states that 6 meters (20 feet) is the maximum total length of a standard SCSI
daisy chain. But, what is sometimes not taken into consideration are the added lengths of wire that
are inside most SCSI sub-systems. If you use 9 inches as an average length, with seven devices
attached, you would be left with only 15 feet of external cabling available before you exceeded
the specification.

Another important consideration is the quality of the cable used. As you add target devices the need
for higher quality cables increases.. Cables that incorporate most or all of the features described in
Table 1 have the greatest chance of offering reliable data-transfers. Table 1 lists some quality
guide lines.

Table 1



High quality SCSI Cable Features 26 Gauge Twisted Pair
Foil and Braided Shielding
Impedance of 100 Ohms (+/- 10%)
20u” Gold on Contacts
20u” Gold on Shell
Extra Heavy Duty Strain Relief
Ferrite Beads
High Quality PVC
Placement of ACK and REQ in the
Center of the Cable




CAUTION

Table 1



High quality SCSI Cable Features 26 Gauge Twisted Pair
Foil and Braided Shielding
Impedance of 100 Ohms (+/- 10%)
20u” Gold on Contacts
20u” Gold on Shell
Extra Heavy Duty Strain Relief
Ferrite Beads
High Quality PVC
Placement of ACK and REQ in the
Center of the Cable




CAUTION
Make sure that the connections are plugged in all the way and that either the screws or locking
tabs are fastened securely before you apply power. SCSI cables should not be plugged in or
removed with the power on. There is always the potential for problems when the SCSI Bus is altered
while the power is left on any device.

To further maintain reliability for any SCSI daisy chain isolate the cables from other wires, and keep
the cables away from other sources of power or data signals. Try to eliminate any electrostatic or
electromagnetic effects that other cables might supply. Also keep the cables from making
excessively sharp bends. The most common reason a cable goes bad is from putting undo strain
on it while making bends.

There are three basic cable connector types that have been accepted by the SCSI community.

50 Pin Centronics
Used in many of the external sub-systems, this is the most popular type of SCSI connector on the
market. It is designed for SCSI-1 and some SCSI-2 applications. It has a maximum data-transfer
rate of around 5 MBytes/Second.

50 Pin SCSI-2
Used mainly in the workstation environment, SUN officially made this the connector of choice for
many of their new lines of SparcStations. These connectors are .050 on center,. half the pitch of the
50 Pin Centronics connector. The maximum data-transfer rate for this type of connector is
10 MBytes/Second.

68 Pin SCSI-2 Wide
Not widely supported yet, this relatively new connector offers twice the data path and increased
data-transfer potential. This connector is also .050 on center, half the pitch of the 50 Pin Centronics
connector. The maximum data-transfer rate for this type of connector is 20 MBytes/Second.