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Electro Industries/GaugeTech
Doc # E107706 V1.25 5-6
For All RS-485 Connections:
Use a shielded twisted pair cable 22 AWG (0.33 mm
2
) or larger, grounding the shield at one end
only.
Establish point-to-point configurations for each device on a RS-485 bus: Connect (+) terminals to
(+) terminals; connect (-) terminals to (-) terminals.
Protect cables from sources of electrical noise.
Avoid both “star” and “tee” connections (see Figure 5.7). No more than two cables should be
connected at any one point on an RS-485 network, whether the connections are for devices,
converters or terminal strips.
Include all segments when calculating the total cable length of a network. If you are not using an
RS-485 repeater, the maximum length for cable connecting all devices is 4000 feet (1219 meters).
RT EXPLANATION:
Termination Resistors are generally used on both ends of longer length transmission lines.
The value of the Termination Resistors is determined by the electrical parameters of the cable.
Use RTs only on Master and Last Slave when connecting multiple meters in a Daisy Chain.
RS-485 communication allows multiple devices to communicate on a bus. The Nexus 1250/1252
Ports 1 to 4 are RS-485 terminals, viable for a distance of up to 4000 feet (1219 m). (Nexus’ Port 1
can be switched between RS-232 and RS-485.) Below is a detail of a 2-wire RS-485 port.
2-Wire RS-485 Port
S - +
Shield
.
.
.
..
Twisted
Pair
Connect (-) to (-) of
next device
Connect (+) to (+) of
next device
Figure 5.6: 2-Wire RS-485 Port Detail