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MINT – IO
REF NO: m25A/om/101
Issue No: 11
Warning:
Failure to follow improper installation practice of RS485 wiring
and power supply wiring may cause failure of IO modules, specifically
communication failures.
2.2 Grounding/Shielding
In most cases, mint-IO modules will be installed in an enclosure along with
other devices which generate electromagnetic radiation. Examples of these
devices are relays and contactors, transformers, motor controllers etc. This
electromagnetic radiation can induce electrical noise into both power and
signal lines, as well as direct radiation into the module causing negative
effects on the system. Appropriate grounding, shielding and other protective
steps should be taken at the installation stage to prevent these effects. These
protective steps include control cabinet grounding, module grounding, cable
shield grounding, protective elements for electromagnetic switching devices,
correct wiring as well as consideration of cable types and their cross sections.
2.3 RS485 Network Wiring
RS485 is designed to be used with a single twisted pair cable. One of the
restrictions of this system is that the common mode voltages of the nodes on
the network should not exceed -7V or +10V. In order to ensure that this
condition is met, it is recommended that the 0V connections on the modules
be connected together. For modules that are far apart, a second twisted pair
should be used as the 0V link. In certain applications where there are strong
possibilities of an earth loop being caused by the 0V link, the link should be
tied to the 0V terminal on each module through a 100ohm resistor, to limit the
earth loop current. Where earth loop problems exist, it may be necessary to
isolate the RS485 network either using optical fiber or an isolated RS485
repeater.
2.3.1 RS485 Cabling Methodology
Method-1, Single Twisted pair, No shield
In this case, “Earth” is ground and it is inexpensive, easy to install. This kind
of cabling is suitable. if conduits are used for communication cables, power
supply cables are not available and environment is free from electrical noise.
This method is not recommended for industrial applications.
Method-2, Shielded single twisted pair + Earth wire
One pair is used for RS-485 communications and extra wire used specifically
for a ground wire.
Method-3, Shielded single twisted pair cable
One pair is used for RS-485 communications and shield is used for return.
Method-4, Shielded twisted pair, 2 pairs
One pair is used for the RS-485 communications and another pair is used for
ground Method 2 to 4 would reduce noise induced through ground potential
differences. This is the preferred option in areas where there is a potential for
high electrical noise or if cabling lacks the cleanliness of conduit or wire trays.
