Seagate STT20000N, Руководство по эксплуатации продукта

Страница: 32 / 70

Chapter 3 Installation
Page 24 STT20000A Product Manual
The cable characteristic impedance should not be less than 90 ohms nor greater
than 140 ohms. A cable characteristic impedance of greater than 100 ohms is
recommended.
To minimize noise and ensure even distribution of terminator power, the minimum
recommended conductor size is 28 AWG (0.08042 mm
2 ).
Electrical characteristics
This section lists measurements of various electrical signals in relation to the single-
ended SCSI connection. For these measurements, SCSI bus termination is
assumed to be external to the SCSI device.
All signals except GROUND and TERMPWR must be terminated at both ends of the
cable. Each signal termination consists of 220 ohms (+ or -5%) to TERMPWR and
330 ohms (+ or -5%) to GROUND and must meet the following specifications:
•
Terminators must supply a characteristic impedance of 100 to 132 ohms.
•
External terminators must be powered by the TERMPWR line, and units
providing terminator power to the cable must have:
V
TERM
= 4.25 to 5.25 VDC
900 milliamps minimum source drive capability
•
When TERMPWR matches the above values, the voltage of released signal
lines must be at least 2.5 VDC.
•
When a driver asserts a line and pulls it to 0.5 VDC, the current available to the
signal line driver may not exceed 48 milliamps. The first two terminators may
only supply 44.8 milliamps of this current.
•
When at least one device supplies TERMPWR, these conditions may be met by
any valid configuration of targets and initiators.
All signals use open-collector drivers. The output characteristics (measured at the
connector of the drive) of signals driven by the STT20000 SCSI minicartridge drive
are:
•
Signal assertion (low-level output voltage): 0.0 to 0.5 VDC at 48 milliamps
sinking
•
Signal negation (high-level output voltage): 2.5 to 5.25 VDC
Signals received by the STT20000 SCSI minicartridge drive have the following
characteristics.
•
Signal assertion (low-level input voltage): 0.0 to 0.8 VDC
•
Signal negation (high-level input voltage): 2.0 to 5.25 VDC
•
Maximum input load (low-level input current): - 0.4 at 0.5 VDC
•
Minimum input hysteresis: 0.2 VDC