DK-RC-MBS-1
User’s Manual r3.3
EN
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16/23
4.6
EIA-485 bus. Termination resistors and Fail-Safe Biasing
mechanism
EIA-485
bus requires a 120Ω terminator resistor at each end of the bus to avoid signal
reflections.
In order to prevent fail status detected by the receivers, which are
“
listening
”
the bus,
when all
the transmitters
’
outputs are in three-state (high impedance), it is also required a fail-safe
biasing mechanism. This mechanism provides a safe status (a correct voltage level) in the bus
when all the
transmitters’
outputs are in three-state. This mechanism must be supplied by the
Modbus Master.
The DK-RC-MBS-1 device includes an on-
board terminator resistor of 120Ω that can be
connected to the EIA-485 bus by using DIP-switch SW4.
Some Modbus RTU EIA-485 M
aster devices can provide also internal 120Ω terminator resistor
and/or fail-safe biasing mechanism (check the technical documentation of the Master device
connected to the EIA-485 network in each case).
