SCSI Termination

SCSI termination is similar to grounding, a necessary step so that the signal is not reflected back from the end of the bus. The SCSI bus is likened to a taut string in that, when one end is vibrated, it causes the vibration to travel through the string until it reaches the other end, and then because it has nowhere to go, the vibration is reflected back to the direction it came from, bouncing from end to end until all the energy is dissipated. This is the same thing that happens with the electrical pulses that go through the SCSI bus; they are reflected back from where they came if proper termination has not been observed. As would be expected, this prevents any actual data from being recognized because of the noise and interference generated.

The electrical wave has to be terminated at the physical ends of the bus, i.e., at the connectors where it meets open air. This is done by installing a terminator at the ends of the bus after all of the actual devices on the chain.

Types of termination:

• Single-ended (SE) — The SCSI controller pushes out signals to all devices using a single data line and each device at the end acts as a ground. Because the signal degrades quickly, SE SCSI is limited to a maximum of 3 m. This is the most common signaling used in PCs.
• High-voltage differential (HVD) — Each device connected to the SCSI bus has a signal transceiver and uses this as a signal booster so that signals can reach further (25 m). This is usually used on servers.
• Low-voltage differential (LVD) — A variation of HVD, but instead of the transceiver being built into the device, it is smaller and built into the SCSI adapter on each device so the voltage required for communication is also lower. Cheaper to implement compared to HVD but only has half the range at about 12 m.