iSMA-B-MAC36NL User Manual
Version 1.4
www.gc5.pl
Page 11 / 51
Figure 6. RS485 communication bus connection
2.3.1
RS485 Grounding and Shielding
The device can be exposed to electromagnetic environment. The electromagnetic radiation
can induce electrical noise into both power and signal lines, as well as direct radiation into the
device causing negative effects to the device functioning. Appropriate grounding, shielding
and the other protective steps should be taken at the installation stage to prevent undesirable
effects. The preventions include making control cabinets grounding, cables shield grounding,
using protective elements for electromagnetic switching devices, using correct wiring as well
as appropriate cable types selection and cable cross-sections.
2.3.2
RS485 Network Termination and Biasing
The transmission line often creates communication problems. These problems include
reflections and signal attenuation.
To eliminate the presence of reflections at the ends of the bus cable, it must be terminated at
both ends with a resistor across the line. The resistor value has to be the same as a
characteristic impedance of the bus cable. Both ends must be terminated since the direction
of propagation is bidirectional. In case of an RS485 twisted pair cable, the termination is
typically 120 Ω.
In the iSMA-B-MAC36NL version there is a built-in 3 position Switch on the back side of the
device (access after removing the back cover) which is dedicated to connecting termination
