Instruction manual

6. MODBUS COMMUNICATION PROTOCOL

IMR01H02-E3

6.4 Slave Responses

(1) Normal response

• In the response message of the Read Holding Registers, the slave returns the read out data and the

number of data items with the same slave address and function code as the query message.

• In the response message of the Preset Single Resister, the slave returns the same message as the

query message.

• In the response message of the Diagnostics (loopback test), the slave returns the same message as

the query message.

• In the response message of the Preset Multiple Resister, the slave returns the slave address, the

function code, starting number and number of holding registers in the multi-query message.

(2) Defective message response

• If the query message from the master is defective, except for transmission error, the slave returns the

error response message without any action.

Slave address

Function code

Error code

Error check CRC-16

Error response message

• If the self-diagnostic function of the slave detects an error, the slave will return an error response

message to all query messages.

• The function code of each error response message is obtained by adding 80H to the function code

of the query message.

Error code Contents

1 Function code error (Specifying nonexistent function code)

2 When any address other than 0000H to 02EEH and 1388H to 14A0H are

specified.

(However, no error returns for any address from 03E8H to 0563H. Therefore,

do not access any of the above addresses.)

3 When the specified number of data items in the query message exceeds the

maximum number of data items available

4 Self-diagnostic error response