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SiAM CRIO v3.0
User Manual
8.3.- Modbus functions
The SiAM CRIO borrad supports the modbus RTU protocol functions described in the
following sections. As frames are limited to 256 bytes, is not possible to read or write
more than a hundred modbus registers in a single transaction. In all examples the slave
address is 128 (0x80).
8.3.1.- Read Holding Registers (0x03)
This function code is used to read the contents of a contiguous block of holding registers.
The following example reads holding register 0x0000:
Request
Field name
Slave address
Function
Starting address (HI)
Starting address (LO)
Number of registers (HI)
Number of registers (LO)
Checksum (HI)
Checksum (LO)
Value (Hex)
0x80
0x03
0x00
0x00
0x00
0x01
0x9A
0x1B
Response
Field name
Slave address
Function
Byte count
Value of register 0x0000 (HI)
Value of register 0x0000 (LO)
Checksum (HI)
Checksum (LO)
Value (Hex.)
0x80
0x03
0x02
0x00
0x00
0x84
0x5A
8.3.2.- Read Input Registers (0x04)
Identical to the previous one. In this example, registers 0x0200-0x0201 are read:
Request
Field name
Slave address
Function
Starting address (HI)
Starting address (LO)
Number of registers (HI)
Number of registers (LO)
Checksum (HI)
Checksum (LO)
Response
Value (Hex.)
0x80
0x04
0x02
0x00
0x00
0x02
0x6E
0x62
Field name
Slave address
Function
Byte count
Value of register
Value of register
Value of register
Value of register
Checksum (HI)
Checksum (LO)
0x0200
0x0200
0x0201
0x0201
(HI)
(LO)
(HI)
(LO)
Value (Hex.)
0x80
0x04
0x04
0x45
0x02
0x00
0x00
0xDE
0x40
The read value of register 0x0200 is 0x4502, and the value of register 0x0201 is 0x0000.
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