CertiGuide to A+ (A+ 4 Real)  9  Chapter 10: The Small Computer System Interface, SCSI

Logical Unit Numbers (LUNs) 1 Passive Termination

Termination¹¹³ is used to eliminate signal reflections along the electrical path of the SCSI bus. Any part of the SCSI bus electrical path that is different from the rest of the path can cause an electrical reflection. These signal re-flections can combine or interact in ways to cause corrupted commands or data. Signal reflections can result in erratic operation or data loss.

There are four types of termination for a SCSI bus:

• Passive Termination:¹¹⁴ This consists of resistors only.
  
  
• Active Termination:¹¹⁵ This type adds a voltage regulator to ensure that the SCSI signals are always terminated to the correct voltage levels. Active ter-mination is recommended when the bus is near the maximum length and when SCSI bus speeds are greater than 10 MHz.
  
  
• Forced Perfect Termination (FPT):¹¹⁶ This is similar to active termination but uses diode clamps to eliminate overshoot and undershoot of signal voltage. Forced Perfect termination should not be used for a differential bus.
  
  
• Multimode Termination: Low-Voltage Differential (LVD) buses require the use of a modified type of termination called multimode. Multimode termination allows single-ended and LVD devices to coexist on the same physical bus. Multimode terminators sense what type of device is being used and terminate the bus accordingly.

Logical Unit Numbers (LUNs) 1 Passive Termination