How To Control; How To Control The Frequency Converter; Function Codes Supported By Modbus Tcp; Modbus Tcp Message Framing Structure - GE AF-650 GP Quick Installation Manual

Opcmbtcp modbus tcp option enclosed drive panel

Hide thumbs Also See for AF-650 GP:

Quick manual (256 pages)

Programming manual (215 pages)

Design manual (212 pages)

Table Of Contents

Table of Contents

5 How to Control

5.1.1 How to Control the Frequency Converter

This section describes codes which can be used in the function and data fields of a Modbus TCP message. For a complete description of all the message fields

please refer to the section Modbus TCP Message Framing Structure.

5.1.2 Function Codes Supported by Modbus TCP

Modbus TCP supports use of the following function codes in the function field of a message

Function

Read holding registers

Write single register

Write multiple registers

Device identification

Function

Function Code

Diagnostics

5.2 Modbus TCP Message Framing Structure

5.2.1 Function Code

The function code of a message frame contains 8 bits. Valid codes are in the range of 1-FF. Function codes are used to send messages between master and slave.

When a message is sent from a master to a slave device, the function code tells the slave what kind of action to perform. When the slave responds to the master,

it uses the function code to indicate either a normal (error-free) response, or that some kind of error occurred (called an exception response). For a normal response,

the slave simply echoes the original function code. For an exception response, the slave returns a code that is equivalent to the original function code with its

most significant bit set to logic 1. In addition, the slave places a unique code into the data field of the response message. This tells the master what kind of error

occurred, or the reason for the exception. Please also refer to the section Function Codes Supported by Modbus TCP and Exception Codes.

5.2.2 Data Field

The data field is constructed using sets of two hexadecimal digits, in the range of 00 to FF hexadecimal. These are made up of one TCP character. The data field

of messages sent from a master to slave device contains additional information which the slave must use to take the action defined by the function code. This

can include items such as coil or register addresses, the quantity of items to be handled, and the count of actual data bytes in the field.

5.2.3 CRC Check Field

Messages include an error-checking field, operating on the basis of a Cyclical Redundancy Check (CRC) method. The CRC field checks the contents of the entire

message. It is applied regardless of any parity check method used for the individual characters of the message. The CRC value is calculated by the transmitting

device, which appends the CRC as the last field in the message. The receiving device recalculates a CRC during receipt of the message and compares the calculated

value to the actual value received in the CRC field. If the two values are unequal, a bus time-out results. The error-checking field contains a 16-bit binary value

implemented as two 8-bit bytes. When this is done, the low-order byte of the field is appended first, followed by the high-order byte. The CRC high-order byte is

the last byte sent in the message

Function Code

3 hex

6 hex

10 hex

2B hex

Sub-function code

Sub-function

Restart communication

Return diagnostic register

Clear counters and diagnostic register

Return bus message count