Rs-485 Cabling Considerations; Rs-485 Loop Topology; Figure 7-5 Proper Shieldc - Siemens MJ-XL Installation Manual

7.2.4 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).
To further protect against external electrical interference, the pair is shielded by a grounded outer covering. The
shield conducts radio frequency interference (RFI) to the ground, thus reducing its effect on the twisted pair.
Concern over RFI and EMI is important given the high electrical current and electrically noisy environment
through which the communication wires are routed.
Follow the rules below to properly install the RS-485 cables.
1. Connect all field devices in a loop technology so that all devices are connected to the master in case of a line
break. The basic loop topology is shown below.
One of the advantages of the loop topology is that if a line is cut, communication is not interrupted. By
completing the loop in the RS-485 cable, one break can be made anywhere in the line without compromising
communication with the master device (RTU). To maintain redundancy, avoid open ended runs of the bus.
Each installation method (see Figures 2-4 to 2-6) has advantages and disadvantages. The loop method requires
more cable that does the single-ended run method. This extra cable is needed to run from the last device on the
run to the master device (RTU). The additional cable adds expense and shortens the total distance the farthest
device can be located from the master device. The advantage of the loop method is in the ability to
communicate with all devices when there is a break in the loop.
The single-ended run allows larger distances between the master device and the farthest slave device, but it
does not allow the master device to communicate with devices on the far side of a break. This method also
requires the use of terminating resistors.
2. A maximum of 32 devices may be connected in a single RS-485 bus with a total cable run no longer than 4000
feet.
3. Always ground the shield at only one end of a cable segment. For all devices, ground the cable shield at one
end only to prevent induced interference that may result from circulating ground currents. If a cable' s shield is
grounded at both ends, a ground lop can exist between the components. This ground loop can result in induced
interference that causes signal distortion. The diagram below shows the proper method for terminating the
shield at the Communications Module connector.
Shield grounded
at other end only
RS-485 Twisted-Pair Cable
From RTU or other device
18
MJ-X
RTU
MJ-X
Figure 7-4 RS-485 Loop Topology
MJ-X Communications Module
Data
Figure 7-5 Proper Shield Connections
Twisted-pair
Cabling
4000 ft max. total
MJ-X
GND
Shield attached
at this end only
RS-485 Twisted-Pair Cable
To next device