SCSI Terminations Information

SCSI terminations are electrical circuits placed at each end of a SCSI cable for impedance matching. They are an important part of any small computer system interface (SCSI) that uses hard drives, CD-ROMs, scanners, tape drives, or other SCSI peripherals controlled by a SCSI bus. SCSI terminations are used to close off both ends of the system that begins with the bus. Many different types of SCSI terminations are available. The most common types are passive, active, active negation, high-voltage differential (HVD), low-voltage differential (LVD), multimode (LVD / MSE), forced perfect terminator (FPT), and high- byte terminator (HB).


SCSI terminations consist of both passive and active products. Passive terminations are the simplest, but least reliable SCSI terminations. They use simple resistor networks to terminate the bus in single-ended SCSI-1 short systems with few connected devices. Passive SCSI terminations are driven by the term power-line on the bus, which has equivalent impedance around 132 ohms. Because the typical impedance of most cables is between 85 and 110 ohms, passive SCSI terminations do not provide good impedance matching. Active terminations are used with single-ended SCSI-2 devices and are compatible with passive SCSI terminations. These SCSI terminations consist of resistor networks driven by voltage regulators inside the termination. The voltage regulator generates a termination voltage from the term power and keeps the terminator’s impedance around 110 ohms.


Active negation terminations, high-voltage terminations, and low-voltage terminations are common types of SCSI terminations. Active negation terminations are used with single-ended SCSI buses. They assert a signal that is required to drive the signal voltage low. With active negation SCSI terminations, the signal is de-asserted or driven high, which allows for faster bus speeds. High-voltage differential (HVD) SCSI terminations are used only in differential systems. They consist of a 330/150/330 resistor network. HVD SCSI terminations are not compatible with single-ended or LVD systems or devices. Low- voltage differential (LVD) SCSI terminations are a special, active termination form defined in the SCSI Parallel Interface 2 and SCSI Parallel Interface 3 documents.


Multimode (LVD/MSE) SCSI terminations are used in low-voltage differential (LVD) systems with multimode transceivers. Depending on the voltage level at the DIFFSENSE pin of the cable, the transceiver automatically chooses LVD or multimode single-ended (MSE) when devices power up. Active LVD/MSE terminations respond to the DIFFSENSE line. If the DIFFSENSE voltage is less than 0.7 V, the termination enters single-ended mode. If the voltage is between 0.9 V and 1.9 V, however, the termination enters LVD mode. 


SCSI terminations include forced-perfect terminations and high-byte terminations. Forced perfect (FP) SCSI terminations are single-ended devices that use diode clamps to compensate for the mismatch between the SCSI cable and the device connected to the bus. The diode clamps force the termination to the correct voltage; however, this termination is not recognized by the SCSI specifications. High-byte (HB) SCSI terminations use the upper data byte on a wide bus (bits 8 to 15). A high-byte termination is used when the SCSI bus terminates in multiple locations.