Appendix
A - 6
A.2.4
Response message
When a slave receives a query except broadcasting, the slave must return a response. Operation of slave can be
defined as following four patterns.
1) Query is receivedand processed properly.
2) Query is unreceived by communication error etc.
3) Although query was received, parity, CRC or LRC error was detected.
4) Although query was received, query cannot be processed by some reasons.
In case of 1), the slave returns the normal response according to the function code in the query.
In case of 2) 3), Since a slave cannot recognize query, it is no response. Therefore, it is necessary to make it a
time-out error by host.
In case of 4), error response (exception response) is returned from a slave. An exception response consists of the
device No., function code and data fields. The device No. of a slave is set to the device No. field like a normal
response. The same function code as query is set to a function code, and the MSB is set to 1, by which a host can
detect an exception response. The exception code which shows the exceptional contents is set to the data field.
An exception code and description are shown in the following table.
Table A.5 Exception code of Modbus protocol
Exception code
Name
Contents
01
Illigal function
The function code received in the query is not supported by the
slave.
02
Illigal data address
The data address received in the query is not supported by the
slave.
03
Illigal data value
A value contained in the query data field is not allowed for the
slave.
04
Slave device failure
Impossible to respond due to device failure
05
Acknowledge
The slave has accepted the request and is processing it, but it takes
time to response.
06
Slave device Busy
The slave is engaged in processing of the last command.
Summary of Contents for EH-150 Series
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