7. MODBUS
IMS01J02-E4
107
7.1.4 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 Register and Diagnostics (Loopback test), the slave
returns the same message as the query message.
In the response message of the Preset Multiple Registers, 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.
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 (An unsupported function code was specified)
2
When written to
read only
data
When any address other than 0000H to 1FFFH is specified
3
When the data written exceeds the setting range
When the specified number of data items in the query message exceeds the
maximum number of data items available
(3)
No response
The slave ignores the query message and does not respond when:
The slave address in the query message does not coincide with any slave address settings.
The transmission parameter of the master does not coincide with that of the slave.
Transmission error such as overrun, framing, parity and etc., is found in the query message.
If data time interval in the query message from the master is following
Modbus mode 1: 24 bits’ time or more
Modbus mode 2: 24 bits’ time
2 ms or more
Slave address
Function code
Error code
Error check CRC-16
Error response message
