Installation, Operation and Maintenance Manual
MAN712 Rev. 9
June 2021
Modbus RTU Interface
Section 8: Modbus RTU Interface
79
8.4.6
Exception Codes
When a master sends a request to a slave, it expects a response. The following cases
may occur:
•
The slave receives the request and returns the normal response.
•
The slave does not receive the request and no response is returned. In this case the
master waits for a certain time (timeout) and then it restarts with a new request.
•
The slave receives the request but is detects a communication error (CRC).
In this case the slave does not return a response; the master waits for a certain
time (timeout) and then it restarts with a new request.
•
The slave receives the request without communication error, but it cannot process
the data. In this case the slave returns an Exception Error message that describes
the error.
The ELBS-20 Modbus RTU Interface has implemented the following Exception Codes:
Table 27.
Exception Code
Name
Description
01
Illegal Function Code
The function code requested is not supported in the slave
device selected.
02
Illegal Data Address
The data address contained in the request message is
not allowable in the slave device, or the combination of
reference number and transfer length is invalid.
03
Illegal Data Value
A value contained in the request data field is not valid for
the slave device.
04
Slave Device Failure
An error occurred while the slave device was performing
the action requested by the request.
06
Slave Device Busy
The slave device is busy. The master should retransmit the
request later.
