SCSI Terminators

SCSI terminators 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 terminators are used to close off both ends of the system that begins with the bus. Many different types of SCSI terminators 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 terminators consist of both passive and active products. Passive terminators are the simplest, but least reliable SCSI terminators. They use simple resistor networks to terminate the bus in single-ended SCSI-1 short systems with few connected devices. Passive SCSI terminators 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 terminators do not provide good impedance matching. Active terminators are used with single-ended SCSI-2 devices and are compatible with passive SCSI terminators. These SCSI terminators consist of resistor networks driven by voltage regulators inside the terminator. The voltage regulator generates a termination voltage from the term power and keeps the terminator’s impedance around 110 ohms.

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

Multimode (LVD/MSE) SCSI terminators 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 terminators respond to the DIFFSENSE line. If the DIFFSENSE voltage is less than 0.7 V, the terminator enters single-ended mode. If the voltage is between 0.9 V and 1.9 V, however, the terminator enters LVD mode. 

SCSI terminators include forced-perfect terminators and high-byte terminators. Forced perfect (FP) SCSI terminators 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 terminators 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.

• SCSI Terminators
  SCSI terminators are an electrical circuit at the end of the SCSI cable designed for impedance matching. With the terminator in place, the electrical signals traversing the cable will not be reflected...
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• Understanding and Testing SCSI Terminators
• Multimode Terminators (LVD / MSE)
  Multimode terminators are used in LVD systems that use multimode transceivers. Depending on the voltage level that appears at the DIFFSENSE pin of the cable, the transceiver automatically chooses...
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Part Numbers for SCSI Terminators