Spectracom 9289, Instruction Manual

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Spectracom Corporation Model 9289
NetClock/NTP Network Time Provider Instruction Manual 2-13
2.7.4 RS-485 Guidelines and Cable Selection
Low capacitance, shielded twisted pair cable is recommended for installations where the RS-
485 cable length is expected to exceed 1500 feet. Table 2-3 suggests some manufacturers and
part numbers for extended distance cables. These cables are specifically designed for RS-422
or RS-485 applications; they have a braided copper shield, nominal impedance of 120 ohms,
and a capacitance of 12 to 16 picofarads per foot.
RS-485 cable may be purchased from Spectracom. Specify part number CW04xxx, where xxx
equals the length in feet.
MANUFACTURER PART NUMBER
Belden Wire and Cable Company
1-800-BELDEN-1
9841
Carol Cable Company
606-572-8000
C0841
National Wire and Cable Corp.
232-225-5611
D-210-1
Table 2-3: Cable Sources for RS-485 Lines Over 1500 Feet
For cable runs less than 1500 feet, a lower-cost twisted pair cable may be used. Refer to Table
2-4 for possible sources. In addition, Category 5 cables may be used for cable runs less than
1500 feet.
MANUFACTURER PART NUMBER
Alpha Wire Corporation
1-800-52ALPHA
5471
Belden Wire and Cable Company
1-800-BELDEN-1
9501
Carol Cable Company
606-572-8000
C0600
Table 2-4: Cable Sources for RS-485 Lines Under 1500 Feet
2.7.5 Connection Method
The RS-485 transmission line must be connected in a daisy chain configuration as shown in
Figure 2-7. In a daisy chain configuration, the transmission line connects from one RS-485
receiver to the next. The transmission line appears as one continuous line to the RS-485 driver.
A branched or star configuration is not recommended. This method of connection appears as
stubs to the RS-485 transmission line. Stub lengths affect the bus impedance and capacitive
loading which could result in reflections and signal distortion.