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5.2 RS-485 Twisted-pair Cable
The following sections provide the information necessary to select and install cable between an MJ-5 Control Panel,
RTU, and other field devices in an RS-485 network.
5.2.1
RS-485 Cable Type
Listed in Table 7-1 are the cable characteristics necessary for proper electrical performance in an RS-485
communications network. Use cable with a shield that provides 100 percent RFI/EMI coverage.
Table 5-1 RS-485 Cable Specifications
Characteristic
Value
Maximum/Typical
Impedance
120 ohms
Typical
Capacitance (pF/ft)
35
Maximum
Cable Size
22 AWG
Typical
DC Resistance
17 ohms/1000 ft
Maximum
Velocity of Propagation
80%
Maximum
5.2.2
RS-485 Cable Grounding
Ground the cable shield for all devices on the network. Ground the cable shield at only one end to prevent induced
interference that may result from circulating ground currents. If a cable shield is grounded at both ends, a ground
loop can exist between the components. This ground loop can cause induced interferences that result in signal
distortion. If there is a ground potential rise between the connected devices, connect the Signal Return (RTN)
between the communication devices. Figure 7-2 illustrates the preferred field device connection method. See section
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for jumper considerations.
Figure 5-2: RS-485 Connection
5.2.3
RS-485 Connector
The required connector for connecting to the MJ-5 RS-232/485 Interface is the Weidmuller Contact Screw Terminal
Block BLT 5.08/8/180 or equivalent. Wire the connector as shown in Figure 7-2. This connector is supplied with the
communications module and is shown in Figure 7-3.
Figure 5-3: Screw Terminal Block Connector
5.2.4
RS-485 Cabling Considerations
The cable used for connecting the Communications Module to other communication devices is a twisted pair of
insulated conductors. The pair is twisted to minimize pickup of random signals between the conductors, particularly
those signals induced by electromagnetic interference (EMI).
12 Gauge Wire (maximum)
Weidmuller Terminal Block
BT 5.08/8/180
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