RLE LD5100 - Manual (Page 53)

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8 Modbus Communication
8.2.2 Function 04: Read Input Registers
To read the LD5100 input values, the master must send a Read Input Registers request packet.
The Read Input Registers request packet specifies a start register and the number of registers
to read. The start register is numbered from zero (30001 = zero, 30002 = one, etc).
Table 8.4 Read Input Registers Packet Structure
Read Registers Request Packet Read Registers Response Packet
Slave Address (1 byte) Slave Address (1 byte)
04 (Function code) (1 byte) 04 (Function code) (1 byte)
Start Register (2 bytes) Byte count (1 byte)
# of registers to read (2 bytes) First register (2 bytes)
Crc Checksum (2 bytes) Second register (2 bytes)
…
Crc Checksum (2 bytes)