Chapter 1 Installation And Set-Up; Modbus Communications Overview; Modbus Network Characteristics; Modbus Protocol - Siemens SIMATIC TI505 User Manual

1.1

Modbus Communications Overview

Modbus Network
Characteristics

Modbus Protocol

Transmission
Modes
1-2 Installation and Set-up
The Modbus system is a Local Area Network (LAN) designed to work in an
industrial environment. It is a master/slave network in which a number of
slave nodes (e.g., Programmable Logic Controllers, or PLCs) are connected
to a Master Node, which is a host computer.
The Modbus protocol determines how messages are passed between the host
computer and the secondary nodes. Up to 247 slaves can reside on a single
network. Each node in that network must have a unique address ranging
from 1 to 247.
Because it is a master/slave network, each request is paired to one response.
The exception to this is broadcast mode, when no response is returned. This
mode is only valid for Modbus functions 5, 6, 8, 15, and 16.
NOTE: Only the master can initiate a message in a Modbus network.
The following two types of messages are used.
The Request/Response type, where a single node is addressed.
The Broadcast/No Response type, where all nodes are addressed but
none respond.
The Modbus message consists of an "envelope" which contains different
types of data. The envelope enables the data to be directed to the correct
address on the network. It also contains information to determine if the
contents were received correctly and instructions about what to do with the
data.
Two modes of data transmission are available for a given network; however,
only one mode can be selected at a given time, and every node on the
network must conform to that mode. Usually, the requirements of the host
computer determine which mode should be selected. The two modes are the
following.
ASCII (American Standard Code for Information Interchange)
RTU (Remote Terminal Unit)
The MODNIM can be set to operate in either mode.
SIMATIC TI505/TI500 MODNIM User Manual