Shielding And Grounding Considerations; Cable Requirements; Network Topology - ABB RS485 Installation And Startup Manual

3.8 Shielding and grounding considerations

The signal ground conductor is often overlooked when ordering cable. An extra twisted
pair must be specified to have enough conductors to run a signal ground. A two-wire
system then requires two twisted pairs.
It is often hard to quantify if shielded cable is required in an application or not. Since the
added cost of shielded cable is usually minimal it is worth installing the first time.

3.9 Cable requirements

The type of wire to use will vary with required length. Wire with twisted pairs and an
overall shield is used most often. The shield is tied to earth ground or chassis, and
typically at one end only (generally at the Modbus Master side). The shield is not to be
used as a signal common or ground. The table below lists typical cable
recommendations.
RS-232
Up to 15m (50ft) virtually any standard shielded twisted pair with drain (Belden 9502 or
equivalent).
RS-422 and RS-485
Up to 1200m (4000ft) 24 AWG twisted pair with foil shield and drain wire on each pair
(Belden 9841 for 2-wire and 9729 for 4-wire or equiv.).

3.10 Network topology

Various kinds of network topologies may be done on the basis of an RS485 Modbus
network.
The Modbus network may be managed by a computer collecting data. Typically this
computer runs an OPC server connected to a plant intranet. OPC client applications may
present this information to any supervision program that will perform the Human machine
interface, data logging, and data setting ...
It is also possible to manage the RS485 Modbus network through a PLC.
8 – Serial interface considerations  RS-485