FANUC Robotics America GFK-1535A - Manual (Page 36)

GFK-1535A Chapter 2 Installation 2-17
2
Using Other Cable Types
The cable types listed in the preceding table are recommended. If the cable types
listed above are not available, the cable selected must meet the following guidelines.
1. High quality construction. Most important is uniformity of cross section along
the length of the cable. Poor quality cable may cause signal distortion, and
increase the possibility of damage during installation.
2. Precision-twisted shielded wire of EIA RS422 standard type, having a uniform
number of twists per unit of length. This type of cable may also be listed as
twinaxial cable, data cable, or computer cable.
3. Relatively high characteristic impedance; 100 to 150 ohms is best; 75 ohms is
the minimum recommended.
4. Low capacitance between wires, typically less than 20pF/foot (60pF/meter). This
may be accomplished by inner dielectrics of foamed type, usually polypropylene
or polyethylene, having a low dielectric constant. Alternatively, the conductors
may be spaced relatively far apart. Lower impedance types have smaller cross-
sections and provide easier wiring for shorter total transmission distances.
5. Shield coverage of 95% or more. Solid foil with an overlapped folded seam and
drain wire is best. Braided copper is less desirable; spiral wound foil is least
desirable.
6. An outer jacket that provides appropriate protection, such as water, oil, or
chemical resistance. While PVC materials can be used in many installations,
Teflon, polyethelene, or polypropylene are usually more durable.
7. Electrical characteristics: cable manufacturers' information about pulse rise time
and NRZ data rate is useful for comparing cable types. The Genius bit consists of
three AC pulses; the equivalent NRZ bit rate is about three times as great.
For assistance in selecting a specific cable type, please consult your local GE Fanuc
application engineer.
Bus Length
The maximum bus length for shielded, twisted-pair cable is 7500 feet. Some cable
types are restricted to shorter bus lengths. In turn, the bus length determines which
baud rate may be selected. Refer to the heading Selecting a Baud Rate.