Honeywell HC900 User Manual page 39

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
Slave
Address
(00 for TCP)
TCP Example 00
RTU Example 01
Revision 13
April 2017
Response message format for function codes 03
Function Byte
Code Count
03 10 42 C8 00 00
03 10 42 C8 00 00
HC900 Process Controller Communications User Guide
Modbus/TCP & Modbus RTU Function Codes
Function Code 03- Read Holding (Data) Registers
Data Data
42 C8 00 00 42 C8 00 00
(100) (100) (100)
42 C8 00 00 42 C8 00 00
(100) (100) (100)
Data Data CRC
(RTU)
42 5D 47 AE
(55.32)
42 5D 47 AE CRC CRC
(55.32)
CRC
(RTU)
31