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5.4 RS485 Interface
Most RS485 buses require termination resistors across the conductor pair. The need for
termination has to be checked for each installation. Especially for high data rates, steep
edges or long cables termination resistors are mandatory. Only both ends of the main
cable, i.e. at the first and the last device, require termination resistors, additional resistors
excessively load the drivers. The resistor value matches the cable´s differential mode
characteristic impedance (in most cases 100 − 120 Ω).
termination has to be checked for each installation. Especially for high data rates, steep
edges or long cables termination resistors are mandatory. Only both ends of the main
cable, i.e. at the first and the last device, require termination resistors, additional resistors
excessively load the drivers. The resistor value matches the cable´s differential mode
characteristic impedance (in most cases 100 − 120 Ω).
At the RS485 bus you need a controller with fail-safe biasing meaning a pull-up and a pull-
down resistor on the cable. The fail-safe biasing provides a known state in which there is no
active driver on the bus. Therefore this is essential regardless of data rates and length of cables.
down resistor on the cable. The fail-safe biasing provides a known state in which there is no
active driver on the bus. Therefore this is essential regardless of data rates and length of cables.
Technical data (for baud rates up to 100 kBps):
Max. cable length:
1200 m (4000 feet)
Max. stub length:
because of reflections stubs should be kept as
short as possible;
exceptions allow a length up to 5 m (16 feet)
short as possible;
exceptions allow a length up to 5 m (16 feet)
Recommendation for the cable:
twisted pair, cable-cross section at least
0.22 mm
0.22 mm
2
(AWG 24) differential-mode
characteristic impedance 100 − 120 Ω
10
rf_prm15_wi_en
V10/09/08
PRM15