PiXtend eIO Hardware Manual
3.3.
Bus Lines
If a RS485 bus is operated over large sockets and in industrial environments (large
currents, many, varying disturbances), the use of shielded cables is always recommended.
For a Modbus compliant design, you must always work with shielded cables. The shield
must be connected to PE (protective earth / ground) at one point, usually at or near the
bus master.
Only in moderate EMC areas - office, laboratory, z.T. in the building services engineering,
within small machines or devices - can be worked also practicably and reliably with
unshielded lines.
Basic specifications for cable lengths and cross sections for the RS485 bus can be found
in chapter 6.3 from page 82 onwards.
A generally applicable solution for the shielding and wiring that is appropriate for every
application is not possible. At this point, we only give tips and suggestions so that you can
assess your specific application.
The shielding of the cables used is connected to PE (protective earth / ground) / FE
(functional earth). Special earthing clamps and shielding pads exist for this, which are
fastened, for example, on a DIN rail - close to the bus master.
Whether the shield is necessary at one or both ends of the line depends on the type of
interference that is to be reduced. A general statement or recommendation is not possible
here. We usually start with the one-sided shield support and adapt it later if necessary. For
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Figure 5: shielded bus cables - shield support
