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18 Commander SK Advanced User Guide
www.controltechniques.com Issue Number: 9
6.1.5 Data encoding
MODBUS RTU uses a 'big-endian' representation for addresses and data items (except the CRC, which is 'little-endian'). This means that when a
numerical quantity larger than a single byte is transmitted, the MOST significant byte is sent first. So for example
16 - bits 0x1234 would be 0x12 0x34
32 - bits 0x12345678L would be 0x12 0x34 0x56 0x78
6.1.6 Function codes
The function code determines the context and format of the message data. Bit 7 of the function code is used in the slave response to indicate an
exception.
The following function codes are supported:
FC03 Read multiple
Read a contiguous array of registers. The slave imposes an upper limit on the number of registers, which can be read. If this is exceeded the slave
will issue an exception code 2.
Table 6-1 Master request
Table 6-2 Slave response
FC6 Write single register
Writes a value to a single 16bit register. The normal response is an echo of the request, returned after the register contents have been written. The
register address can correspond to a 32bit parameter but only 16 bits of data can be sent.
Table 6-3 Master request
Code Description
3 Read multiple 16bit registers
6 Write single register
16 Write multiple 16bit registers
23 Read and write multiple 16bit registers
Byte Description
0
Slave destination node address 1 through 247, 0 is
global
1 Function code 0x03
2 Start register address MSB
3 Start register address LSB
4 Number of 16bit registers MSB
5 Number of 16bit registers LSB
6 CRC LSB
7 CRC MSB
Byte Description
0 Slave source node address
1 Function code 0x03
2 Length of register data in read block (in bytes)
3 Register data 0 MSB
4 Register data 0 LSB
3+byte count CRC LSB
4+byte count CRC MSB
Byte Description
0 Slave node address 1 through 247 0 is global
1 Function code 0x6
2 Register address MSB
3 Register address LSB
4 Register data MSB
5 Register data LSB
6 CRC LSB
7 CRC MSB
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