PiXtend eIO Hardware Manual
Connection Notes
With RS485 interfaces it comes again and again to confusion because of the designation
of the connections. The names A & B may vary depending on the manufacturer. As a rule,
incorrect wiring does not lead to permanent problems or defects, as is the case with
PiXtend controllers and eIO's.
In the case of PiXtend eIO we adhere to the designations of the "MODBUS over Serial
Line specification V1.02": D1, D0, C. In addition, we additionally use the names of the
transceiver chip manufacturers with A, B, GND printed on the devices.
A and B names from the integrated circuits (chips) are always exactly the opposite as from
the RS / EIA-485 standard. This circumstance is the reason for all the confusion.
For PiXtend V1.x, V2 -L and eIO devices, the names of A & B are always the same - so A
is connected to A and B to B.
For a Modbus compliant bus configuration, GND must be routed between the individual
bus subscribers via the "C" (common) connections and connected to the PE (protective
earth / ground) at one point. The connection is usually made anyway, which therefore
causes no additional effort.
The connection between "C" and "PE" is usually made at or near the bus master. In
addition, shielded cables must be used with Modbus, and the shield must also be
connected to PE.
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Figure 34: Principle circuit diagram: connection of the RS485 interface
