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Modbus RTU Function Codes

02/13 Modbus® RTU Serial Communications User Manual 13

3.1 Function Code 01 – Read Digital Output Status

Description

Function code 01 (0X references) is used to read a Digital Output’s (DO) ON/OFF status of the slave device

in a binary data format. All binary data transferred using function code 01 is mapped into bytes.

The specific number of Dos available in an instrument or available via one Function Code 01 message is

instrument-model specific.

Broadcast is not supported.

Query

The query message specifies the starting DO and the quantity of coils to read. Dos are addressed starting at

zero: DO 1 through 16 are addressed as 0 through 15 respectively.

Query message format for function code 01

Slave

Address

Function

Code

Starting

Address

High

Starting

Address

Low

Number

High

Number

Low

CRC CRC

Example:

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

02 01 00 00 00 07 CRC CRC

Response

The DO status in the response message is packed as one DO per bit of the data field. Status is indicated as: 1

= ON; 0 = OFF. The LSB of the first data byte contains the DO addressed in the query. The other Dos

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

If the returned DO 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 01

Slave

Address

Function

Code

Byte

Count

Data Data … CRC CRC

Example:

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

02 01 01 42 CRC CRC

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

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

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

1 2 ... 14 15 16 17 18 19 20 21 22 23 24 ... 75 76

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