Read Holding Registers (Code = 0x03)

The function code 0x03 (Read Holding Registers) allow to read informations, randomly, from the main PLC table

The PLC module acts like SERVER and the request should be prepared by a host system working as CLIENT. The PLC table is 16-bits word aligned array of general purpose data registers. The register address is in the range from zero to the maximum number of registers of your PLC (See specific technical data). For example, if my PLC has 1000 words, my address range is from 0 to 999

Field	Lenght	Description
Start Address	2 Bytes	This field keeps the "first location address" that will be readed from the PLC table. The field represent a 16bits-aligned entity and can span from 0 to the maximum table size
Quantity of Registers	2 Bytes	This is number of registers that will be readed starting from the first specified address

REQUEST (SENT BY CLIENT SYSTEM)
Fields	Mnemonic	Description	Lenght	Sample Data	Sent bytes and order
MBAP		Transaction Identifier	2 Bytes	0x2356	23H 56H
		Protocol Identifier	2 Bytes	0x0000	00H 00H
		Lenght	2 Bytes	0x0006	00H 06H
		Unit Identifier	1 Byte	0x05	05H
MODBUS Request	FC	Function Code	1 Byte	0x03	03H
		Start Address	2 Bytes	0x0240	02H 40H
		Quantity of Registers	2 Bytes	0x0004	00H 04H

RESPONSE (SENT BY SERVER PLC)
Fields	Mnemonic	Description	Lenght	Sample Data	Sent bytes and order
MBAP		Transaction Identifier	2 Bytes	0x2356	23H 56H
		Protocol Identifier	2 Bytes	0x0000	00H 00H
		Lenght	2 Bytes	0x0006	00H 06H
		Unit Identifier	1 Byte	0x05	05H
MODBUS Response	FC	Function Code	1 Byte	0x03	03H
		Content of register 0x0240	2 Bytes	0x1234	12H 34H
		Content of register 0x0241	2 Bytes	0x5678	56H 78H
		Content of register 0x0242	2 Bytes	0x9ABC	9AH BCH
		Content of register 0x0243	2 Bytes	0xDEF0	DEH F0H