Using An Rs-485 Bus; Basic Feature Of Rs-485Bus; Mode Of Connection And Terminal Resistance - Comelit MPTZ37IR Manual Del Usuario

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Using an RS-485 Bus

1. Basic Feature of RS-485bus

According to the industry bus standard, The RS-485 bus is half-duplex communication bus with
a typical impedance of 120Ω, whose maximum load capacity is 32 payloads (including the
master device and the controlled device).

2. Mode of Connection and Terminal Resistance

2.1 The RS485 bus requires that parallel connections should be used between the devices
with 120Ω terminal resistances connected at the both ends.
Fig 26 Fig 27
2.2 120Ω terminal resistance is connected as shown in fig 27.
120Ω terminal resistance is available in the circuit board and the connection is shown as
following:
When needing to connect 120Ω resistance, toggle the 8
th
bit of DIP switch SW2 to "ON" . This
will connect the 120Ω resistance to the circuit.
29