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A Appendix: Modbus Interface WirelessHART Fieldgate SWG70

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A Appendix: Modbus Interface

A.1 Introduction

A.1.1 Modbus protocol

Modbus is a quasi-industrial standard developed some years ago by Gould-Modicon and provides a

messaging service that may run on a variety of physical layers. For Fieldgate SWG70 there are two

possibilities for connecting Modbus:

• Modbus RTU (also known as Serial) can be connected point-to-point to the RS-485C interface

as described in Chapter 5.4.

• Modbus TCP (also known as Modbus TCP/IP) can be connected to an Ethernet Interface as

described in Chapter 5.3.

The Modbus protocol exchanges data in a master-slave relationship. Each slave has a unique

address, and the data are identified by their location in the slave address register. Certain

characteristics of the Modbus protocol are fixed, such as the frame format, frame sequences,

handling of communications errors, exception conditions and the functions performed. Other

characteristics are user selectable; these include transmission medium, baudrate, character parity,

number of stop bits, and transmission modes. Chapter 8.3 describes how both Ethernet and Serial

interfaces can be set up. The contents of the data carried by the protocol are also freely selectable,

i.e. nothing is said about strings, integers, floating-point numbers etc.

The Modbus protocol controls the query and response cycle between master and slave devices. Only

the master can initiate a transaction. A query and response may involve only a single slave, or it may

be in the form of a broadcast, in which case the slaves do not answer. The query is contained in a

frame that includes the address of the intended receiver, what this slave is to do, data needed to

perform the action, and a means of checking for errors. The slave checks if errors have occurred and

performs the desired action. After the action is performed the slave builds the response and returns

it to the master. The master can send another message to any slave as soon as it receives a valid

response or after a user-selected time interval. This "timeout" period has to be selected on the master

device and depends on the slave response time.

Data can be exchanged in two transmission modes: ASCII (American Standard Code for Information

Interchange) and RTU (Remote Terminal Unit). The major differences between them are the type

of error check performed on the message and the number of characters used. Fieldgate SWG70

supports RTU only. Modbus offers several read, write and test functions, each identified by a code

number. They are designed as control commands for sensors and actuators, e.g. coils, inputs, input

registers, holding or output registers, diagnosis and test reports, programs, polling control and reset.

For Modbus TCP the serial frame is simply inserted into the Ethernet data frame. In addition, not

all codes are implemented.

Modbus overview

Modbus RTU Modbus TCP

Standard compliance "Modbus over Serial Line" V1 "Modbus over TCP" V1

Physical layer RS-485 Ethernet

Transmission mode RTU (binary mode) TCP

Baudrates 1200 bit/s, 2400 bit/s, 4800 bit/s,

9600 bit/s, 19200 bit/s, 38400 bit/s,

57600 bit/s, 115200 bit/s

100 MBit/s

Partiy Odd, Even, None –

Stop bits 1; 1.5; 2 –

Polling address 1...247 –

Port No. – 502

Capabilities • Input registers starting at Modbus address 30013

• HART Command 3 dynamic variables mapped into input registers

• 2 input registers map a single HART dynamic variable

• 32bit HART floating point format used

• Status information mapped on dedicated input registers

1 2 ... 87 88 89 90 91 92 93 94 95 96 97 ... 123 124

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