Avago Technologies LSI53C140 - Manual (Page 33)

SCSI Signal Descriptions 2-15
Ver. 2.1 Copyright © 1998–2001 by LSI Logic Corporation. All rights reserved.
2.2.11.4 Clock (CLOCK)
This is the 40 MHz oscillator input to the LSI53C140. It is the clock
source for the protocol control state machines and timing generation
logic. This clock is not used in any bus signal transfer paths.
2.2.12 SCSI Termination
The terminator networks provide the biasing needed to pull signals to an
inactive voltage level, and to match the impedance seen at the end of
the cable with the characteristic impedance of the cable. Terminators
must be installed at the extreme ends of each SCSI segment, and only
at the ends. No SCSI segment should ever have more or less than two
terminators installed and active. SCSI host adapters should provide a
means of accommodating terminators. The terminators should be
socketed, so they may be removed if not needed. Otherwise, the
terminators should be disabled by means of software.
LSI Logic requires the use of multimode terminators because these
terminators provide both LVD and SE termination, depending on what
mode of operation is detected by the DIFFSENS pins. HVD requires a
different termination configuration. The use of active termination is highly
recommended.