PiXtend V2 -L- Hardware Manual
Warning!
Exceeding the specified maximum values may result in transmission
errors, malfunctions and component defects.
Switching between RS232 and RS485 transceivers is possible via the Raspberry Pi
GPIO18. If the GPIO18 is configured as an input or drives a LOW level as output, then
RS232 is active. If a HIGH level is output, RS485 is active.
RS232 is the default setting for the PiXtend system.
Connection Notes
In the RS485 bus, a terminating resistor of 120 Ω must be connected at the ends of the
line structure (first and last bus node). If PiXtend is one of these bus ends, the jumper
must be set to "ON". In series with the 120 Ω resistor is a 1 nF capacitor. This prevents
unnecessary current flow (power dissipation) at the quiescent level.
The "bias network" built into PiXtend stabilizes the levels on the bus when it is "un-driven"
or "floating". Without such a network, so-called "phantom characters" can be received by
the bus subscribers, which can result from disturbances / vibrations on the bus lines
(unwanted change of the logic level).
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Figure 83: Principle circuit diagram: Connection of the RS485 interface
