Modbus Troubleshooting PDF

[Omaelk]

This PDF offers Modbus Troubleshooting. Modbus protocol defines a simple Protocol Data Unit (PDU) independent of the underlying communication layers. The mapping of Modbus protocol on specific buses, or networks, can introduce some additional fields on the application data unit (ADU). Modbus is a request/reply protocol and offers services specified by function codes. Function codes are elements of MODBUS request/reply PDUs.

Product Line

EcoStruxure Building Operation, TAC IA Series, TAC Vista, Andover Continuum, Satchwell BAS & Sigma

Table of Contents

Modbus Introduction – 1

Brief Modbus History – 1

Modbus Protocol Description – 1

Understanding Modbus – 1

Modbus Memory Sections – 1

Registers – 2

Function Codes – 3

Polling – 4

Modbus Remote Terminal Unit (RTU) – 5

Introduction – 5

RTU Packet – 6

Transmission Mode – 7

RS – 485 – 7

Modbus TCP – 8

Introduction – 8

MBAP Header – 8

Supporting Layers – 9

Excerpts

Understanding Modbus – Modbus Memory Sections, Registers, Function Codes, and Polling

“Modbus has four memory sections that consist of four data types. The four sections of memory hold specific data in registers … Registers are also known as “Words” equal to sixteen bits or two bytes … The interpretation of a register is based on how data is specified (e.g. analog or digital).” – Page 1

How Modbus Broadcast Messages Work

“Broadcast requests are writing commands. All devices must accept a broadcast writing request.” – Page 5

Client Devices Requesting Data From a Server Device, Unicast or Broadcast; RTU Packets

“The function code indicates to the server what kind of action to perform. The function code is followed by a data field that contains request and response parameters.” – Page 6