Wiring - Macurco Modbus RS-485 User Instructions

Wiring

Signal Wire
A MODBUS over 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 RS485-MODBUS must use a balanced
pair (for A-B) and a third wire (for the Common). For RS485-MODBUS, Wire Gauge must be chosen sufficiently wide to permit the
maximum length (1000 m or 3281ft). AWG 24 is always sufficient for the MODBUS Data. Category 5 cables may operate for RS485-
MODBUS, to a maximum length of 600 m 1968.5 ft. For the balanced pairs used in an RS485-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.
Topology
An RS485-MODBUS 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 RS485-MODBUS is also
possible.
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