Special Signals
96122 • Revision: F Chapter 2 SCSI Bus Physical Description 11
Special Signals
The library supports two special SCSI bus signals:
■ Differential Sense (DIFFSENS)
■ Terminator Power (TERMPWR)
Differential Sense
The differential sense (DIFFSENS) is a DC voltage level that distinguishes among
the two SCSI alternatives: single-ended, and LVD:
The DIFFSENS signal helps prevent damage to the SCSI bus and other equipment
when the SCSI device is incompatible with the SCSI bus. Some potential for device
damage still exists, however, depending on the type of incompatibility. The
following table (TABLE 2-2) shows the effects of different types of incompatibility.
Terminator Power
The library is capable of providing terminator power on the SCSI bus. All devices
supporting the differential alternative (LVD) must have the ability to provide
terminator power (TERMPWR) with the following characteristics:
Note – Industry standards dictate that no more than three devices should provide
terminator power on the bus. This ensures that voltage on the bus stays high (+5
VDC) without over-driving the signal or overloading the bus.
Jumpers on the library enable terminator power.
Single-ended: -0.35 V to +0.5 V
LVD: +0.7 V to +1.9 V
TABLE 2-2 SCSI Device Types into SCSI Bus Types
If you plug this device type
into this SCSI bus type . . . Single-Ended (SE) Bus LVD Bus
Single-ended (SE) device No effect.
This is a proper connection.
The entire bus will run in SE
mode (with SE restrictions).
LVD device The LVD device will run in
SE mode.
No effect.
This is a proper connection.
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