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2.5 External terminal resistance
According to the actual situation, a serial port side need external gateway 120 Ω
terminal resistance.RS485 bus in the case of no trunk to support a maximum of 32
nodes, node and node between the "Daisy chain" connection mode, in the
communication cable at both ends need to add terminal resistance, the resistance
value is required to be approximately equal to the transmission cable characteristic
impedance. In short distance transmission, no final resistance is required, that is, no
final resistance is generally required below 300 meters. The final resistance is
connected at the ends of the transmission cable.
Gateway in field application, if the RS485 bus distance, big field interference will
need to add 120 Ω on both ends of the RS485 bus terminal resistance, in order to
prevent the reflection of serial signal.
Note: 120 Ω resistance attached to the package, pay attention to check.