Macurco MRS-485 Modbus RS-485 Adapter
REV – 1.0
[34-2900-0019-7]
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3.4
Wiring
3.4.1
Signal Wire
A serial Line Cable should be shielded for best performance. The shield should be connected on each detector at
SHD terminal and connected to a ground terminal or chassis only at one end of the bus. An RS-485 must use a
balanced pair (for A & B) and a third wire (for the Common). For RS-485, Wire Gauge must be chosen sufficiently
wide to permit the maximum length (1,000m or 3,281ft). AWG 24 is always sufficient for the Data. Category 5
cables may operate for RS-485, to a maximum length of 600 m 1968.5 ft. For the balanced pairs used in an RS-
485-system, wire with a characteristic impedance of higher than 100 Ohms may be preferred, especially for
19200 and higher baud rates.
Note
: It is recommended to always use twisted wires to reduce noise and allow for reliable data communication
over greater distances. Use at least 3-conductor wire with one twisted pair providing a pair for signal (A & B) and
Common (COM) connections.
For best performance use shielded 3-conductor wire with one twisted pair providing a pair for signal (A & B),
Common (COM) and shield ground (SHD) connections.
3.4.2
Topology
An RS-485 configuration without repeater has one trunk cable, along which devices are connected, directly
(daisy chaining) or by short derivation cables. The trunk cable, also named “Bus”, can be long. Its two ends must
be connected on Line Terminations. (see Line Termination - End of Line Resistor section). The use of repeaters
between several RS-485 is also possible.
Summary of Contents for BRS-485
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