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Input / Output 4-19MN1904
Final slave s hown with
terminating resi stor, T
R
,
typical value 120Ω.
Network slave
T
R
RX-
DGND
RX+
TX+
TX-
RX-
DGND
RX+
TX+
TX-
RX-
DGND
RX+
TX+
TX-
Twisted pai rs
T
R
Connect overall shield
to connec tor backshell.
Network slave
Network
master
Figure 12 - 4-wire RS485 multi-drop connections
Each TX/RX network requires a termination resistor at the final RX connection, but
intermediate devices must not be fitted with termination resistors. An exception is where
repeaters are being used which may correctly contain termination resistors.
Termination resistors are used to match the impedance of the load to the impedance of the
transmission line (cable) being used. Unmatched impedance causes the transmitted signal to
not be fully absorbed by the load. This causes a portion of the signal to be reflected back into
the transmission line as noise. If the source impedance, transmission line impedance, and
load impedance are all equal, the reflections (noise) are eliminated. Termination resistors
increase the load current and sometimes change the bias requirements and increase the
complexity of the system.