Deister electronic RDL150 RS232, Wiring And Installation Instruction

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deister electronic GmbH 30890 Barsinghausen Germany 7
V19/07/04
RDL150 RS485/RS232
4. RS485 Interface
Most RS485-buses require termination resistors across the conductor pair. The need for termina-
tion has to be proofed for each installation. Especially for fast transitions, high data rates or long
cables the resistors are absolutely necessary. Only both ends of the main cable require termination
resistors, additional resistors excessively loads the driver. The resistor value matches the cable's
differential-mode characteristic impedance (100-120 Ohm).
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 and therefore it is essential, independent from data rates and length of the cable.
(Bus with RS485 interface)
Technical data (for baud rates up to 100 kHz):
Max. cable length: 1200m (4000 feet)
Max. stub length : because of reflections stubs should be as short as possible
Exceptions allow a length up to 5 m (16 feet)
Recommendation
for the cable:
twisted pair, cable-cross-section at least 0,22mm² (AWG 24)
differential-mode characteristic impedance 100-120 Ohm