Quadlogic MiniCloset-5c, Руководство по установке, Страница: 83 / 117

Appendix
© COPYRIGHT 2009
Quadlogic Controls Corporation
79
RS-485 O
VERVIEW
Quadlogic devices may sometimes use a RS-485 interface to construct a multi-point
communications network. The RS-485 interface is connected in a 4-wire full-duplex mode
and is capable of handling 32 transmitters along with 32 receivers. In a four-wire network it
is necessary that one node be a master node and all others be slaves. The network is
connected so that the master node communicates to all slave nodes and all slave nodes
communicate only with the master node.
G
UIDELINES FOR
P
ROPER
W
IRING OF A
RS-485 N
ETWORK
Cable Selection1
Selecting data cable for a RS-485 system is important because intermittent communication
problems are often caused by marginal cable and can be difficult to troubleshoot. The most
important parameters that dictate the type of cable that will be used are Characteristic
Impedance, Shunt Capacitance, and cable length or transmission run.
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.
Cable Length (Transmission Run)
Typical RS-485 systems have a maximum transmission run of 4000 feet. The total
transmission run will start from the first unit up to the last unit in the data link
network.
The type of cable used for RS-485 is typically a twisted-pair wire which is simply a pair of
wires with equal lengths and is twisted together. A twisted-pair wire helps prevent radiated
EMI and it also reduces the effects of received EMI. Because the two wires are close together
and twisted, the noise received on one wire will tend to be the same as that received on the
second wire. This type of noise is referred to as "common-mode noise." As RS-485 receivers
are designed to look for signals that are the opposite of each other, they can easily reject
noise that is common to both.
Recommended wires include Delco 43902, Belden 3087A, and Belden 9842.
1
http://www.arcelect.com/485info.htm