Mastervolt MasterBus - Entering the Values into Modbus; Modbus Function Code 23

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6 September 2014 / MasterBus – Modbus Interface / EN

6.5 Entering the values into Modbus

After having written down the values needed, you

have to enter these into your Modbus system. The

next example shows how to enter the values and

how to communicate with the chosen variable

“Override” of MasterBus device “INT DC Relay”.

6.5.1 Modbus function code 23

The Modbus to MasterBus interface uses the

Modbus function 23 communication protocol.

See the Modbus Application Protocol Specification

V1.1b at www.modbus.org for more details.

The dataframe tables below describe the variables

used in Modbus function 23 (0x17) Read/Write

Multiple Registers Protocol.

Request data frame

Address field Function code (Function 23) Data (Read Starting Address, etc. ) CRC (Error check)

1 Byte 1 Byte 21 Bytes 2 Bytes

Request

Variable Size Value

Bus address 1 Byte Variable

Function code 1 Byte 0x17 (Fixed)

Read Starting Address 2 Bytes 0 (Fixed)

Quantity to Read 2 Bytes 6 (Fixed)

Write Starting Address 2 Bytes 0 = read / 1 = write

Quantity to Write 2 Bytes 6 (Fixed)

Write Byte Count 1 Byte 12 (Fixed)

IDAL 5 bit value 1 Byte Variable

IDB 3 Bytes Variable

TabNr 2 Bytes Variable

Index 2 Bytes Variable

Value 4 Bytes Variable

CRC 2 Bytes Calculated

Response data frame

Address field Function code (Function 23) Data (Read Starting Address, etc. ) CRC (Error check)

1 Byte 1 Byte 13 Bytes 2 Bytes

Response