Topology is a term that refers to the shape or structure of things. In the
computer world, it usually refers to the way that devices are connected or arranged. The
SCSI interface uses a bus topology. This means that all devices are daisy-chained
linearly in a long line. This is identical to how a thin Ethernet (10base2) network is set
up (the cables and signals aren't the same of course, I am referring to how the devices
are logically linked together in a line). This is sometimes called a SCSI chain as
well.
When setting up a SCSI bus, it is imperative that each device be in a straight line.
This means that each device (including the host adapter) is connected to either one or two
other devices, and never more than that. The two devices at the end of the bus must be terminated, either internally or
externally. The bus should never be connected in a loop, star or other formation.
For a two-device bus, the topology typically looks like this:
Terminator -- Device A -- Device B -- Terminator
For a four-device bus, it is as follows:
Terminator -- Device A -- Device B -- Device C -- Device D -- Terminator
It does not matter which device is located where on the chain, and any of the devices
can be either internal or external, but the terminators must be at the ends. The
terminators can be either explicit devices or part of the final devices on the cable. So
in the second example, if Device D had internal termination, it could be enabled instead
of using a separate terminator on that end of the cable. Obviously, having the internal
devices at one end of the bus, and the external devices at the other end, with the host
adapter in the middle, is going to be the setup that makes the most sense for those that
are using both internal and external drives. If the host adapter is placed in the middle
of the chain, it must have its termination disabled, otherwise the devices past it on the
chain will not function. (The exception is if the host adapter implements multiple
channels or segments. If so, each channel or segment must be separately terminated; see here more.)
If using a cable with more connectors than you have devices, it is acceptable to leave
some of the connectors unused, but they should be left in the middle of the cable, with
the terminators still at the end of the cable. Terminating a SCSI chain while leaving
connectors "dangling" is not a good idea, as this can create signaling problems.
So the following is legal:
Terminator -- Device A -- (unconnected) -- Device B -- Device C -- (unconnected) --
Terminator
But the following is not legal:
Terminator -- Device A -- Device B -- Device C -- Terminator -- (unconnected) --
(unconnected)
Some SCSI chains, particularly those used for faster transfer modes or LVD signaling,
work better if the last device on any SCSI cable is connected to the last connector on the
cable, the one closest to the terminator. Also, evenly spacing devices out over the bus is
preferable from a reliability standpoint.Thus, the following would probably be a better
configuration than prior example, even though that one is technically legal:
Terminator -- Device A -- (unconnected) -- Device B -- (unconnected) -- Device C --
Terminator
Next: Number of Devices
