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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Small Computer Systems Interface (SCSI) | SCSI Configuration ]

SCSI Bus Topology

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


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