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Modbus/TCP & Modbus RTU Function Codes
Function Codes 03- Read Holding (Data) Registers
26 HC900 Hybrid Controller Communications User Guide Revision 4
9/03
master station requests only one register at an address of a floating point value then half of a float will be
returned.
The Modbus RTU protocol has a single byte count for function code 03, therefore the Modbus RTU
protocol can only process up to 63 floating point and 127 16-bit integer values in a single request.
Example:
PV, Remote SP, Working SP, and Output where PV=100.0, RSP=100.0, WSP=100.0, and
Out=55.32 as floating point values where AI #1 = 100.0 and AI #2 = 55.32
Response message format for function codes 03
Slave
Address
(00 for TCP)
Function
Code
Byte
Count
Data Data Data Data CRC
(RTU)
CRC
(RTU)
TCP Example 00 03 10 42 C8 00
00
(100)
42 C8 00
00
(100)
42 C8 00
00
(100)
42 5D 47
AE
(55.32)
RTU Example 01 03 10 42 C8 00
00
(100)
42 C8 00
00
(100)
42 C8 00
00
(100)
42 5D 47
AE
(55.32)
CRC CRC