There are three forms of SCSI termination recognized: Active, Passive and the most complex one: Forced-Perfect.
This is a more stable form of terminating SCSI cables. Active terminators control the impedance at the end of the SCSI bus by using a voltage regulator, not just the power supplied by the interface card.
A passive terminator sits on the bus to minimise reflections at the end of the cable. Passive terminators simply provide impedance close to that of the cable.
The terminator is “passive” because it doesn’t do any work to regulate power for termination; it relies on the interface card to provide steady power.
Of all SCSI terminators, this is the most complex. A cable with a forced-perfect terminator can actually change its impedance to compensate for variations along the bus. Forced-perfect terminators force the impedance of the cable to match each device through diode switching and biasing.
Learn more: Introduction to SCSI technology.