Termination

Termination simply means that each end of the SCSI bus is closed, using a resistor circuit. If the bus were left open, electrical signals sent down the bus could reflect back and interfere with communication between SCSI devices and the SCSI controller. Only two terminators are used, one for each end of the SCSI bus. If there is only one series of devices (internal or external), then the SCSI controller is one point of termination and the last device in the series is the other one. If there are both internal and external devices, then the last device on each series must be terminated.Types of SCSI termination can be grouped into two main categories: passive and active. Passive termination is typically used for SCSI systems that run at the standard bus clock speed and have a short distance, less than 3 feet (1 m), between the devices and the SCSI controller. Active termination is used for Fast SCSI systems or systems with devices that are more than 3 ft (1 m) from the SCSI controller.Another factor in the type of termination is the bus type itself. SCSI employs three distinct types of bus signaling. Signal ling is the way that the electrical impulses are sent across the wires.Single-ended (SE) - The most common form of signaling for PCs, single-ended signaling means that the controller generates the signal and pushes it out to all devices on the bus over a single data line. Each device acts as a ground. Consequently, the signal quickly begins to degrade, which limits SE SCSI to a maximum of about 10 ft (3 m).High-voltage differential (HVD) - The preferred method of bus signaling for servers, HVD uses a tandem approach to signaling, with a data high line and a data low line. Each device on the SCSI bus has a signal transceiver. When the controller communicates with the device, devices along the bus receive the signal and retransmit it until it reaches the target device. This allows for much greater distances between the controller and the device, up to 80 ft (25 m).Low-voltage differential (LVD) - A variation on the HVD signaling method, LVD works in much the same way. The big difference is that the transceivers are smaller and built into the SCSI adapter of each device. This makes LVD SCSI devices more affordable and allows LVD to use less electricity to communicate. The downside to LVD is that the maximum distance is half of HVD -- 40 ft (12 m).Both HVD and LVD normally use passive terminators, even though the distance between devices and the controller can be much greater than 3 ft (1 m). This is because the transceivers ensure that the signal is strong from one end of the bus to the other.