ICP GDT RS Series, User Manual

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B.4.2 SCSI Termination
In order to ensure a flawless and interference-free signal transmission on the SCSI bus and
to minimize the detrimental effects of external noise generators, both ends of the SCSI ca-
ble have to be terminated. The SCSI specification prescribes different termination methods
for SE and LVDS SCSI bus systems.
There are two alternative termination modes for Single-Ended SCSI bus systems: The pas-
sive termination and the active termination, also known as Alternative-2 termination. The
passive termination consists of a 220 Ohm pull-up and a 330 Ohm pull-down resistor for
each signal. Today, the passive termination is mostly used in systems with synchronous
data transfer rates not exceeding 5 MB/sec, which is rather slow. The active termination cir-
cuit consists of a 110 Ohm precision-resistor per signal and a common 2.85 Volt voltage
regulator. Thus, all signals are actively pulled up to a certain level. The active termination
provides much better signal quality and significantly reduced liability to noise.
LVDS SCSI bus systems allow only active LVDS termination. The termination circuitry must
automatically recognize, if the bus is operated in SE or LVDS mode and adjust itself ac-
cordingly.
All Wide/Ultra2 and Ultra 160 SCSI channels of the ICP Controllers are equipped with a
LVDS compliant termination. The additional Wide/Ultra SCSI channel of the Wide/Ultra2
RN Series Controllers uses an active SCSI bus termination. The voltage for the termination
circuitry is supplied either by the SCSI device itself, or by the TERMPWR line(s) of the SCSI
bus.
The new Wide/Ultra2 and Ultra 160 SCSI devices are delivered without termination. I.e., the
SCSI bus cannot be terminated on the last device of the bus, but needs an external termi-
nation which allows SE and LVDS operation (see B.4.4 "ICP SCSI Accessories").
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Always use active SCSI bus termination on SE busses.
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Always use multimode termination on SE/LVD busses.
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Do not use SCSI devices with passive SCSI bus termination (e.g., CD-ROMs) for the ter-
mination of the SCSI cable.
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Always terminate only the two ends of a SCSI cable.
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The TERMPOWER (TPx) jumpers on the ICP Controller PCB should always be set.
The onboard termination of the ICP Controller can be changed within the ICP RAID Console
or the ICP RAID Navigator. The default setting is "Auto". In this mode the ICP Controller
analyses which connectors are occupied and adjusts its termination accordingly. In "Off"
mode the termination of the ICP Controller is always off. In "On" mode the termination is
always on.
In addition to these "soft-switches" the onboard termination can be also set by jumpers on
the ICP Controller PCB.
B.4.3 SCSI ID
All participants on the SCSI bus must have a unique identification number, that is, each
number can only be used once on a given cable. Each SCSI device is uniquely addressed
through its SCSI ID.
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All participants of a SCSI bus must have a different SCSI ID.