Product Guide | Issue 1.5 | GP Gateway
Page 12
Appendices
The GP Gateway
8
Appendices
Appendix 1
8.1
Recommended wiring Practice for RS-485 Networked Devices.
Introduction
RS-485 transmits digital information between multiple locations. RS-485 is designed to transmit this information
over significant lengths, and 1000 meters are well within its capability. The distance and the data rate with which
RS-485 can be successfully used depend a great deal on the wiring of the system. RCS guidelines and
recommendations are provided within this document to ensure robust and reliable RS-485 networks.
Cable
RS-485 is designed to be a balanced system. Simply put, this means there are 2 wires, (other than ground), that
are used to transmit the signal. The system is called balanced, because the signal on one wire is ideally the exact
opposite of the signal on the second wire. In other words, if one wire is transmitting a high, the other wire will be
transmitting a low, and vice versa.
Although virtually any cable is capable of transmitting RS-485 signals, good wiring practice dictates that
TWISTED PAIR
type cable is used.
As its name implies, a twisted pair is simply a pair of wires that are of equal length and are twisted together.
Using an RS-485-compliant transmitter with twisted-pair wire reduces two major sources of problems for
designers of high-speed long-distance networks: radiated EMI and received EMI.
RCS suggest the use of Belden type cable (or similar) such as 9501 / 2, with a characteristic impedance of 120
ohms. The RCS SM1020 gateway devices have termination resistors incorporated, however an end of line
terminator (120 Ohms) is recommended to be fitted on the last device on a zone. The two wire RCS system is
polarity conscious and the + - connections must remain consistent throughout the network.
Maximum Number of RS-485 controllers on a Network
RCS system manager (RS-485) devices and gateways are provided with 1, 4 or 8 channels (Zones) of RS-485.
Each one of these zones can accommodate 32 individual control devices.
It is good practice to ensure that cable lengths are kept to the minimum required without compromising network
integrity. The wiring configuration should be of a Multi-Drop (
daisy chain)
fashion whereby a single twisted pair
cable is terminated at the system manager / Gateway and run to each control device in turn using either the on-
board “in and out” RS-485 connectors or the supplied communications terminals as in the case of Williams
devices, Eden Controller and pack controllers etc. To avoid impedance mismatches and signal reflections and
distortions a
Star Point
wiring configuration must not be used.
