HP 3000/9x9KS Series, Service Manual

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8-6 SCSI and I/O
Termination/Term Power
1. The SCSI bus requires termination at both ends of the bus. The host adapters generally supply on
board terminators and should be located at one end of the bus. The multifunction I/O host bus
adapters are the current exception since they are designed to reside in the middle of the SCSI bus.
Therefore, buses associated to these multifunction I/O adapters are terminated on the two ends of the
associated internal and external buses. Since an external connection may not be present, insure that
the ACTIVE terminators supplied with the product are installed on the external connection at the
back of the system.
2. When possible, put devices that supply termination power at both ends of the bus. The host adapters
will always supply termination power which is required for proper bus operation. However, noise
immunity can be improved by placing another source of termination power at the other end of the
bus. Most peripheral devices are optionally capable of supply termination power. (See the
appropriate peripheral descriptions in this section.)
Termination Characteristics and Power
Terminator power will be supplied by the Initiators and may be supplied by any or all connected devices.
HP recommends that terminator power be supplied by the Host Bus Adapter (HBA) Initiator and the last
device on the SCSI Bus to help decrease interface error rates and provide better balance to the TERMPWR
contacts. Recommended current limiting is 1.5 amps. with a maximum of 5 amps. (The limiting device may
be fused or resetable.)
Single-Ended
TERMPWR = 4.25 to 5.25 vdc. with 900 ma source drive capability for the 50 pin Cable, and 1500 ma
source drive capability for the 68 pin cable.
Differential
TERMPWR = 4.25 to 5.25 vdc. with 600 ma source drive capability for the 50 pin cable, and 1000 ma
source drive capability for the 68 pin cable.
Normal Operation
1. Do not connect or disconnect a device on an active SCSI bus or to any bus that has devices supplying
termination power. Shut down the system and turn off all external SCSI devices before connection or
disconnecting products.
2. Do not power on or off a device on an ACTIVE SCSI bus.
WHY?
a. Noise can be injected which may cause devices on the bus to be reset or cause the system to reset the
bus do to detection of an error. A reset interrupts the normal operation of the bus and can affect system
performance.
b. The action can cause data corruption.
c. Some devices do not meet the specified standards when doing power on (C1512A and 7980S)