Despatch Protocol 3 Modbus Communications Instrukcja Użytkownika Pobierz pdf (Strona 9)

Modbus Programming Manual ABOUT THIS MANUAL

Version 2 9

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Inter-message

gap

Address

1 character

Function

1 character

Data

n characters

CRC Check

2 characters

1.4.6. Supported Modbus Functions

Modbus defines several function types. The following types are supported by this instrument:

Function Code

(decimal)

Modbus Meaning

Description

03 / 04

Read Holding/Input registers

Read current binary value of specified number of

parameters at given address. Up to 64 parameters

can be accessed with one Query.

Write Single Register

Writes two bytes to a specified word address.

Diagnostics

Used for loopback test only.

16 (0x10 hex)

Write Multiple Registers

Writes up to 253 bytes of data to the specified

address range.

23 (0x17 hex)

Read/Write Multiple Registers

Reads and Writes 253 bytes of data to the specified

address ranges.

1.4.7. Function Descriptions

The following is interpreted from the Modbus Protocol Description obtainable from

http://www.modbus.org/. Refer to that document if clarification is required.

In the function descriptions below, the preceding device address value is assumed, as is the

correctly formed two-byte CRC value at the end of the QUERY and RESPONSE frames.

Function 03 / 04 - Read Holding/Input Registers

Reads current binary value of data at the specified word addresses.

QUERY

Function

Address of 1

Word

Number of Words

03 / 04

RESPONSE

Function

Number of Bytes

First Word

Last Word

03 / 04

In the response the “Number of Bytes” indicates the number of data bytes read from the

instrument. E.g. if 5 words are read, the count will be 10 (A hex). The maximum number of

words that can be read is 64. If a parameter does not exist at one of the addresses read, then a

value of 0000h is returned for that word.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 ... 51 52

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