The newest SCSI cables are designed to implement the newest SCSI signaling method, low voltage differential (LVD). Since LVD is a different method of
signaling than regular, single-ended (SE) SCSI, the signals
used on these cables are quite different than those of SE cables. There are also some
differences in terms of the physical cables as well.
First, let's talk about the signals. LVD cables have their signals organized into pairs
just like SE cables. However, instead of each pair consisting of a signal and a signal
return (ground), each pair consists of the positive and negative complements of the
signal. There is also a change to the function of one cable conductor: a special signal
called DIFFSENS, which is used to control differential signaling.
Other than these small changes, the signals for LVD narrow cables are the same as for
SE narrow cables, and the signals for LVD wide cables are the same as for SE wide cables.
Rather than repeat two large tables full of signals with nearly-identical versions, I
would refer you to the tables for narrow and wide cables respectively. As you look at those tables, remember
that for LVD, each "SIGNAL RETURN" is replaced with the "positive"
complement of the active-low regular signal. So for example, cable conductor #15 for an
LVD "A" cable carries the signal "+DB(7)", and cable conductor #57 for
an LVD "P" cable carries the signal "+REQ". For narrow ("A")
cables, DIFFSENS replaces one ground signal on cable conductor #21; for wide
("P") cables it is cable conductor #31.
Note: I should point out
that I mention narrow LVD cables primarily for "completeness"--they are defined
in the SCSI standards but have never been widely used. LVD is generally only implemented
for wide devices.
The physical cables used for LVD SCSI chains are also different from single-ended
cables, despite the fact that the connectors used are the same for single-ended. This is
really a different matter than the changes in signaling; since the wires don't know what
signals they are carrying, in theory one should be able to use the same cables
for LVD as for SE. In practice, however, the greater demands placed by the high speed used
in LVD signaling means that problems can occur if you try to use SE cables for LVD
implementations.
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An internal, 68-wire, 5-connector LVD cable. Note the
distinctive "loose" twisted pair wiring between the connectors.
The circuit board at bottom right is an integrated LVD/SE
terminator. (Incidentally, one of the connectors
is hidden behind the terminator and hard to see.) |
For external cables, you want to use cabling that is specifically rated for LVD use;
these cables have been designed and tested for LVD applications, even if they look the
same as their single-ended cousins. As with regular wide cables,
they may be found with either high density or very high density connectors. Internal LVD
cables are actually very different from SE cables. The reason is that to improve signal
integrity, internal LVD cables typically don't use regular flat ribbon cabling. Instead,
they use so-called "Twist-N-Flat" cabling, where adjacent pairs of wires
are twisted between the connectors, and the wires "flatten out" where the
connectors attach. See the discussion of cabling types for
more on this kind of wiring.
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Detail of the cable pictured above, showing one of the
high density
connectors, along with a flat section of the cable where the connector
attaches, and the twisted pairs of the cable on either side. |
Next: Single Connector Attachment (SCA, SCA-2)
