Cable Selection for RS-422 and RS-485 Systems
RS-485
Quick Guide - Just the Essentials on a Single Piece
of Paper
RS-485
Problem Solver - Trouble Shooting Guide Walks You
Step by Step
Making Sense of the Specifications
Selecting data cable for
an RS-422 or RS-485 system isn't difficult, but often gets
lost in the shuffle of larger system issues. Care should be
taken, however, because intermittent problems caused by marginal
cable can be very difficult to troubleshoot.
Beyond the obvious traits
such as number of conductors and wire gauge, cable specifications
include a handful of less intuitive terms.
Characteristic Impedance
(Ohms): A value based on the inherent conductance, resistance,
capacitance and inductance of a cable that represents the
impedance of an infinitely long cable. When the cable is cut
to any length and terminated with this Characteristic Impedance,
measurements of the cable will be identical to values obtained
from the infinite length cable. That is to say that the termination
of the cable with this impedance gives the cable the appearance
of being infinite length, allowing no reflections of the transmitted
signal. If termination is required in a system, the termination
impedance value should match the Characteristic Impedance
of the cable.
Shunt Capacitance (pF/ft):
The amount of equivalent capacitive load of the cable, typically
listed in a per foot basis. One of the factors limiting total
cable length is the capacitive load. Systems with long lengths
benefit from using low capacitance cable.
Propagation velocity (%
of c): The speed at which an electrical signal travels in
the cable. The value given typically must be multiplied by
the speed of light (c) to obtain units of meters per second.
For example, a cable that lists a propagation velocity of
78% gives a velocity of 0.78 x 300 x 106 = 234
x 106 meters per second.
Plenum Cable
Plenum rated cable is fire
resistant and less toxic when burning than non-plenum rated
cable. Check building and fire codes for requirements. Plenum
cable is generally more expensive due to the sheathing material
used.
The RS-422 specification
recommends 24AWG twisted pair cable with a shunt capacitance
of 16 pF per foot and 100 ohm characteristic impedance. While
the RS-485 specification does not specify cabling, these recommendations
should be used for RS-485 systems as well.
It can be difficult to quantify
whether shielding is required in a particular system or not,
until problems arise. We recommend erring on the safe side
and using shielded cable. Shielded cable is only slightly
more expensive than unshielded.
There are many cables available
meeting the recommendations of RS-422 and RS-485, made specifically
for that application. Another choice is the same cable commonly
used in the twisted pair Ethernet cabling. This cable, commonly
referred to as Category 5 cable, is defined by the EIA/TIA/ANSI
568 specification. The extremely high volume of Category 5
cable used makes it widely available and very inexpensive,
often less than half the price of specialty RS-422/485 cabling.
The cable has a maximum capacitance of 17 pF/ft (14.5 pF typical)
and characteristic impedance of 100 ohms.
Category 5 cable is available
as shielded twisted pair (STP) as well as unshielded twisted
pair (UTP) and generally exceeds the recommendations for RS-422
making it an excellent choice for RS-422 and RS-485 systems.
For more information on serial communications,
including RS485 systems issues, we recommend Serial
Port Complete by Jan Axelson.
|
