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Modbus RTU Function Codes

02/13 Modbus® RTU Serial Communications User Manual 15

3.2 Function Code 02 – Read Digital Input Status

Description

Function code 02 (1X references) is used to read a Digital Input’s (DI) ON/OFF status of the slave device in

a binary data format. All binary data transferred using function code 02 is mapped into bytes.

The specific number of inputs available in an instrument or available via one Function Code 02 message is

instrument-model specific.

Broadcast is not supported.

Query

The query message specifies the starting input and the quantity of inputs to read. Inputs are addressed

starting at zero: Input 1 through 16 are addressed as 0 through 15 respectively

Query message format for function code 02

Slave

Address

Function

Code

Starting

Address

High

Starting

Address

Low

Number

Inputs

High

Number

Inputs

Low

CRC CRC

Example:

Read inputs number 1 to 7 from slave at address 02.

02 02 00 00 00 07 CRC CRC

Response

The input status in the response message is packed as one input per bit of the data field. Status is indicated

as: 1 = ON; 0 = OFF. The LSB of the first data byte contains the input addressed in the query. The other

inputs follow toward the high order end of this byte, and from low order to high order in subsequent bytes.

If the returned input quantity is not a multiple of eight, the remaining bits in the final data byte will be

padded with zeros (toward the high order end of the byte). The byte count field specifies the quantity of data

bytes returned.

Response message format for function code 02

Slave

Address

Function

Code

Byte

Count

Data Data … CRC CRC

Example:

Inputs number 2 and 7 are on, all others are off.

02 02 01 42 CRC CRC

In the response the status of inputs 1 – 7 is shown as the byte value 42 hex, or 0100 0010 binary. Input 8 is

the MSB of this byte, and input 1 is the LSB. Left to right, the status of input 7 through 1 is: ON-OFF-OFF-

OFF-OFF-ON-OFF. Input #8 was not requested and so bit #7 or the MSB was padded with a 0.

1 2 ... 16 17 18 19 20 21 22 23 24 25 26 ... 75 76

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