PiXtend eIO Hardware Manual
There are different topologies for networks. However, star, tree, ring and mesh networks
are not suitable for RS485 and therefore not for PiXtend eIO.
RS485 is always set up as a line or in a bus structure.
All participants within a PiXtend eIO network need the same ground reference. Without a
common ground, there can be transmission problems and, in the worst case, a defect in
the RS485 drivers. In case of doubt, please contact PiXtend support via email with the
most accurate description, sketch and wiring diagram possible at
even before such problems arise.
The GND connection (GND / C) of the RS485 interface is not suitable for such
equipotential bonding. Cables on these connections are only used to increase the
immunity to interference and are optional (not optional for Modbus). Compensation
currents, as an example between different plant components in the industry, must never be
routed via the GND / C connections of the RS485 interface. Exactly so these connections
are also not intended or suitable as a shield connector of cables.
Another common mistake has to do with the termination resistors (termination). Either no
terminators are used or they are in the wrong place on the network. Also, too many (more
than two) termination resistors adversely affect data transfer. Correctly, two resistors are
120 Ω - one at each end of the line structure.
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Figure 4: Bus topology – RS485 – typical mistakes
