August 2015
24
Quick Start Guide
6.5 Modbus output
The Modbus output is a Modbus RTU signal using RS-485. Follow these cable
recommendations for RS-485 interface (Modbus over serial line).
Cable characteristics
Bus cable
Bus to be connected device to device. For example, daisy chained (not star
connected).
Maximum 4000 feet depending on speeds, cable, and loads.
Cable shield must be grounded at only one point.
Due to the use of galvanically isolated Modbus connections, a third common wire
is not necessary for this product. If a 3-conductor cable is used, the third wire
should be left unterminated and insulated from ground.
Derivations (spurs)
Avoid derivations (spurs) when possible. If required, derivations from the bus
must be as short as possible (65 feet maximum).
Termination
A single 120 ohm terminator should be placed at each physical end of the bus (at
the two most remote bus devices) to minimize reflections in the transmission
cable. Do not place terminators on a spur connection.
Modbus wiring
The Modbus signal is a 24VDC active output.
Wire terminal 1 (B/D1) and terminal 2 (A/D0). See Figure 22.
Typ e
Shielded twisted pair cable with 2 conductors and a
drain wire, or Ethernet cable of Cat 5/5e/6
Conductor gauge
20–24 AWG for lengths up to 1000 feet
16–20 AWG for lengths up to 4000 feet
Characteristic impedance 100–130 ohm
Conductor-to-conductor capacitance <30 pF/ft
Conductor-to-shield capacitance <60 pF/ft
Voltage rating 300 V/600 V
Recommended insulation material PVC (<1000 ft) or PE (≥1000 ft)
To Next Page
Loading...
