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Modbus RTU Function Codes

16 Modbus® RTU Serial Communications User Manual 02/13

3.3 Function Codes 03/04 – Read Input Registers

Description

Function code 03 (4X references) or Function code 04 (3X references) is used to read the binary contents of

input registers in the slave referenced in Appendix A. Function codes 3 and 4 are not restricted to inputs.

They may transmit alarm status, control parameters, etc.

The specific supported registers available in an instrument or available via one Function Code 03/04

message is instrument-model specific. When a master station requests a register that is not supported by the

specific device the slave will respond with zeros for that register.

If a request is made to an address that does not exist in the map in Appendix A, the instrument is to honor

that request and return zeros. This behavior will greatly enhance the bandwidth on the link vs. making

several different requests for non-contiguous data elements. (i.e. Consider a device that contains AI #1 and

AI #3 and for some reason AI #2 is an invalid request.) The contiguous method would allow the read of AI

#1 through AI #3 and the data location for AI #2 would be zeros.

Broadcast is not supported.

Query

The query message specifies the starting register and quantity of registers to be read. Registers are addressed

starting at zero: registers 1-16 are addressed as 0-15.

Query message format for function code 03/04

Slave

Address

Function

Code

Starting

Address

High

Starting

Address

Low

Number

Addresses

High

Number

Addresses

Low

CRC CRC

Example:

Read analog inputs #1 and #2 in addresses 1800-1803 as floating point values from a slave at

address 02.

02 04 18 00 00 04 CRC CRC

1 2 ... 17 18 19 20 21 22 23 24 25 26 27 ... 75 76

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