Avago Technologies LSI21040 - Manual (Page 35)

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Detailed Installation Procedure 2-17
2.2.6 SCSI Bus Termination
The devices making up the SCSI bus are connected serially (chained
together) with SCSI cables. The first and last physical SCSI devices
connected on the ends of the SCSI bus must have their terminators
active. All other SCSI devices on the bus must have their terminators
removed or disabled. Remember that the LSI21040 is also on the SCSI
bus—its termination is automatically enabled when it is connected to the
end of the bus.
Important: To utilize Ultra2 and faster SCSI performance, you must
only have LVD devices on the bus. Do not mix any SE
devices with LVD devices or the entire bus will drop to SE,
limiting bus performance to Ultra SCSI levels.
The peripheral device terminators are usually set with jumpers, resistor
modules, or with a switch on the peripheral. Refer to the peripheral
manufacturer’s instructions and to the user’s manual for your computer
for information on how to identify the terminator type/setting for each
device and how to set/change it.
Caution: The autoenable/disable sensing feature on the LSI21040
may enable termination erroneously if it is directly cabled to
another SCSI device or host adapter using the same
sensing method. The LSI21040 senses the presence of
SCSI devices by detecting the ground signal on
conductor 50 of the 68-pin SCSI cable and conductor 22 on
the 50-pin SCSI cable.
The LSI21040 automatically controls SCSI bus termination for three
different bus configurations, depending on how it is connected (see
Figure 2.1 ). The three bus configurations are:
•
Section 2.2.6.1, “Internal Bus Connections”
•
Section 2.2.6.2, “External Bus Connections”
•
Section 2.2.6.3, “Internal and External Bus Connections”
Termination on the LSI21040 for these three different bus configurations
is discussed below.