Figure 3-7: RS-485 Resistor DIP Switches
A. Input terminal connection
B.
Ethernet Port 1
C. Ethernet Port 2
D. Terminal connection 1
E. Terminal connection 2
F. DIP Switches
G. PoE disabled
H. Reset to defaults
I. 1.5 kΩ pull-down resistor
J. 120 Ω terminating resistor
K. 1.5 kΩ pull-up resistor
Switches 1 and 3 are connected to pull-up and pull-down resistors for Modbus connection.
Switch 1 is for the Tx + (A) line and Switch 3 is for the Rx – (B) line. These 1.5 kΩ resistors
are used to prevent noise from being interpreted as valid communications during periods
when no actual communications are occurring. Only one set of pull-up and pull-down
resistors should be active on the RS-485 data bus at time.
Switch 2 is connected to a 120 Ω terminating resistor for Modbus connection. This resistor
is used to dampen signal reflections on long cable runs. RS-485 specifications indicate that
the data bus should be terminated at both ends (Figure 3-7).
Installing the Gateway Reference Manual
May 2020 00809-0600-4410
30 Emerson.com
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