7.2.8 Exception condition responses
The master device expects a normal response when it sends a request to the frequency inverter. A
request by the master can result in one of four reactions:
• If the frequency inverter receives the request without any transmission errors, it can process it and
send a normal response.
• If the frequency inverter does not receive the request due to a transmission error, it will not send a
response. The master will check the conditions for time monitoring of the request.
• If the frequency inverter receives the request and identifies a transmission error (parity, LCR, CRC,
…), it will not send a response. The master will check the conditions for time monitoring of the re-
quest.
• If the frequency inverter receives the request without any transmission error, but cannot process
it, e.g. because an unknown parameter is to the read, it will send an exception response containing
information about the type of error.
The exception condition response contains two fields which are different from normal responses:
Function code field:
In a normal response, the frequency inverter will return the function code of the original request. All
function codes have 0 as the most significant bit (MSB); their values are less than the hexadecimal
value of 0x80. In an exception condition response, the frequency inverter will set the most significant
bit of the function code to 1. This will increase the hexadecimal value of the function code in an ex-
ception condition response by 0x80 compared to the value of a normal response. With the most signif-
icant bit in the function code set to the new value, the master can identify the exception response and
analyze the exception condition code in the data field.
Data field:
In a normal response, the frequency inverter will send data or statistical values in the data field (re-
quested information) . In an exception condition response, the frequency inverter will send an excep-
tion condition code in the data field. This code indicates the cause of the exception condition.
The exception condition codes generated by the frequency inverter are listed in Chapter 7.2.9
“Exception condition codes”.
38
ACU
Modbus/TCP 10/13
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