Instructions

Intesis
TM
INMBSSAM001R000
User’s Manual r1.3 EN
© HMS Industrial Networks S.L.U - All rights reserved
This information is subject to change without notice
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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 INMBSSAM001R000 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 Master 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).