5. Interfaces
5.1 RS485
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 absolutely mandatory. Only both ends of the
main cable, i.e. at the first and the last device, require termination resistors, additional re -
sistors excessively load the drivers. The resistor value matches the cable´s differential mode
characteristic impedance (in most cases 100...120 Ω).
Attention!
This termination resistor (terminator, 120 Ω) is factory built-in into all UDL
250 and has to be deactivated if necessary (see also chapter 9.1.2 “Basic
Setup” –> “enable 485 termination” within the configuration menu “Web-
Config”). The factory setting of this termination resistor is “on”.
At the RS485 bus you need fail-safe biasing containing a pull-up and a pull-down resistor
on the cable which are both built-in into the UDL 250. The fail-safe biasing provides a
known state in which there is no active driver on the bus and is therefore absolutely
mandatory regardless of data rates and length of cables. The pull-up and pull-down resistors
can only be activated together with the termination resistor (within WebConfig).
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Wiring and Installation UDL 250
