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Modbus RTU Function Codes

02/13 Modbus® RTU Serial Communications User Manual 17

Response

The register data in the response message are packed as two bytes per register. For each register, the first

byte contains the high order bits and the second contains the low order bits.

The floating point values require two consecutive registers. A request for a single floating point value must

be for two registers. The first 16 bits of the response contain the IEEE MSB of the float value. The second

16 bits of the response contain the IEEE LSB of the float value. (See Section 2.3.) If the master station

requests only one register at an address of a floating point value, the slave may respond with an exception

with illegal data address code.

The Modbus RTU protocol has a single byte count for function codes 03 and 04, therefore the Modbus RTU

protocol can only process up to 64 floating point and 127 integer values in a single request.

Response message format for function codes 03/04

Slave

Address

Function

Code

Byte

Count

Data Data … CRC CRC

Example:

Analog inputs #1 and #2 as floating point values where AI #1 = 100.0 and AI #2 = 55.32

02 04 08 42 C8 00 00 47 AE 42 5D CRC CRC

1 2 ... 18 19 20 21 22 23 24 25 26 27 28 ... 75 76

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