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Modbus/TCP & Modbus RTU Function Codes
Function Codes 04 - Read Input Registers
28 HC900 Hybrid Controller Communications User Guide Revision 4
9/03
The Modbus RTU protocol has a single byte count for function code 04, therefore the Modbus RTU
protocol can only process up to 63 floating point values in a single request.
Example:
Analog inputs #1 and #2 as floating point values where AI #1 = 100.0 and AI #2 =55.32
Response message format for function codes 04
Slave
Address
(00 for TCP)
Function
Code
Byte
Count
Data Data CRC
(RTU)
CRC
(RTU)
TCP Example 00 04 08 42 C8 00
00
(100)
42 5D 47
AE
(55.32)
RTU Example 01 04 08 42 C8 00
00
(100)
42 5D 47
AE
(55.32)
CRC CRC