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RS485 / Modbus RTU Wiring Standards

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This document from DECK Monitoring suggests Engineering Standards for RS-485/Modbus RTU network wiring.

DECK Monitoring provides information about installation in good faith, but DECK is not responsible for installation. Installation and
electrical wiring should be done by a licensed electrician who has experience working with energy monitoring equipment

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Contents:

Introduction
Cable Configuration
Network Wiring Method
Network Layout
Cable Termination (general)
Cable Termination (RS485/Modbus RTU Line Termination Resistor)
Cable Termination (RS485/Modbus RTU Line Bias)

Overview

Several cables and wiring methods are referenced by installers when deploying Modbus communications networks. Engineers at DECK Monitoring have reviewed information from many sources to create the following recommendations for a standard and robust wiring method. Many of the more common problems associated with RS485 network stability can be mitigated by following these recommendations for wiring, termination and line bias. The items in this document are presented as points of interest to designers and installers of Modbus networks.

DECK Monitoring recommends this cable and network wiring method in an attempt to provide a stable bus for Modbus RTU communications. We recognize that environmental noise at some project sites can lead to issues that may require additional hardware and labor.

Cable Configuration

Please refer to the table below for cable recommendations from DECK Monitoring:

Wiring_1.png

 

Network Wiring Method

DECK Monitoring recommends that you observe each of the following standards for network wiring:

• Devices shall be networked as a single bus.
• Network shall originate from a single Bus Master device.
• Network shall include cable termination per Modbus protocol specifications.
• Network wiring shall be executed with cable approved by DECK Monitoring.
• Devices identified by DECK Monitoring Engineering as inducing noise into the Modbus network shall require each require one (1) approved RS-485  Optical Isolator.

Network Layout

Please refer to the diagram below for correct Modbus network layout:

Wiring_2.png

 

Cable Termination (general)

Exact method for cable termination and conductor landing of Modbus networks will vary depending on whether or not device termination points possess independent DC REF/ COMMON and SHIELD/GROUND landing terminals. Please refer to the table below and the reference diagrams on the following pages:

Wiring_3.png


Figure 2: Modbus Wiring for Devices with Independent DC REF/COMMON 
& SHIELD/GROUND Terminals

Wiring_4.png


Figure 3: Modbus Wiring for Devices Without Independent DC REF/COMMON & SHIELD/GROUND Terminals

Wiring_5.png


Figure 4: Some Slave Devices Feature Independent DC REF/COMMON & SHIELD/GROUND Terminals, Other Devices Do Not

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Figure 5: Some Slave Devices Feature Independent DC REF/COMMON & SHIELD/GROUND Terminals, Other Devices Do Not, and Network is in a
High RF Environment

Wiring_7.png

 

Cable Termination (RS485/Modbus RTU Line Termination Resistor)

The Modbus Standard for Serial Communication (http://www.modbus.org) states that line termination resistors must be placed near each end of the bus:

“A reflection in a transmission line is the result of an impedance discontinuity that a travelling wave sees as it propagates down the line. To minimize the reflections from the end of the RS485 cable it is required to place a Line Termination near each of the 2 ends of the Bus.”

Modbus specifies that a network not requiring line bias shall use a 120Ω ½W resistor between the differential pair. Some devices provide integral termination resistors. Before installing termination the installer should verify the absence of integral termination in the devices at each end of the
Modbus network.

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Networks in a high RF environment may require the use of Line Bias (see next section: CABLE TERMINATION, RS485 / MODBUS RTU LINE BIAS) to stabilize communications. 

 

Cable Termination (RS485/Modbus RTU Line Line Bias)

When the RS485 network is not being actively driven by a device the differential pair is more susceptible to interference and noise. Correct termination of the shield drain to a dedicated ground terminal or chassis should provide noise rejection. In environments where excessive interference and noise still compromises network stability, line biasing (the use of pull-up/pull-down resistors on the differential pair) is required. Individual equipment manufacturers may or may not provide facilities to bias the network. As such DECK Monitoring Engineering directs installers and technicians to reference the applicable installation documents for the equipment in these environments. In the event that none of the devices on the network provide the requisite bias facilities, an approved optical isolator may be used.

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