Electro Industries/GaugeTech Nexus 1250, Installation & Operation Manual

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Electro Industries/GaugeTech
Doc #: E107-7-06-121
5-6
For All RS-485 Connections:
n
Use a shielded twisted pair cable 22 AWG (0.33 mm 2 ) or larger, grounding the shield at one end
only.
n Establish point-to-point configurations for each device on a RS-485 bus: Connect (+) terminals to
(+) terminals; connect (-) terminals to (-) terminals.
n
Protect cables from sources of electrical noise.
n Avoid both “star” and “tee” connections (see Figure 5.6). 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.
n
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).
n
Connect devices using the diagrams on the following pages. A correct connection requires 1/4 watt
resistors connected to the (+) and (-) terminals of each device at the end of a straight-line bus. The
line impedance of the cable and the resistor should match. Generally, a 100- or 120-Ohm resistor
will work.
n
RS-485 communication allows multiple devices to communicate on a bus. The Nexus’ Ports 1 to 4
are RS-485 terminals, capable 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
Courtesy of NationalSwitchgear.com