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5.2 Read Holding Register – Function Code 03
5.2.1 Read Holding Register Query
The Read Holding Register Query obtains the contents of up to eight holding
registers in the addressed slave.
Note. The data start register must contain the offset address of the first
register to be accessed, e.g. to start at register 1 the data start register must
contain 00 (00H).
Broadcast mode is not supported by Function Code 03.
Example. Read three holding registers from slave (01) starting at holding address
01 (process variable input).
Register Register Data No. Data No. Error
Address Function Start Start of Registers of Registers Check (CRC-16)
Offset High Offset Low High Coils Low Field
01 03 00 00 00 03 05 CB
5.2.2 Read Holding Register Response
The addressed slave responds with its address and function code, followed by the
information field. The information field contains one byte describing the quantity of
data bytes to be returned. Two bytes are used to return each register requested,
the first byte containing the high order bits and the second the low order bits.
Example
PV input (two registers) – 270
PV decimal places – 1
Byte Holding Holding Holding Error
Address Function Count Register 01 Register 02 Register 03 Check
(CRC-16)
High Low High Low High Low Field
01 03 00 00 00 01 09 00 01 49 75
…5 MODBUS FUNCTIONS
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